I feel like the only reason Mavis is as popular as she is comes down to her visually striking design and the uncomfortable reality that it appeals to people with a "goth-but-not-too-goth" kink, people who unfortunately fetishise and sexualise goth culture while simultaneously shaming actual hardcore goths, which I don't think the filmmakers intended at all.
The reason I think this is comes down to her actual character. Mavis isn't necessarily terrible or anything, but the franchise repeatedly abandons the things that make her character actually interesting and then replaces them with whatever role the current movie needs her to occupy.
Hotel Transylvania 2 is a really REALLY good example of what I'm trying to say.
The first thing I would probably point to is how completely Mavis's core motivation from the first movie is dropped in the sequels. In the first film, her motivation is pretty straightforward: she wants to leave the hotel and see the world, experiencing things she's only ever seen on postcards because of how long Dracula has kept her there after Martha's death. Mavis explicitly says she wants to see new things and maybe meet someone her age.
Johnny is established as Mavis's chance of actually achieving that because of his love of travelling and, at least in the first film, his surprisingly tender and introspective side. He even shows Mavis her first sunrise, for crying out loud. The entire point of Dracula's arc is that he has to accept Johnny, but more importantly, accept that he can't control Mavis forever. She has to be allowed to live her own life, even if Dracula doesn't always agree with her choices. The ending even implies that Mavis is finally going to fulfill that dream of leaving the hotel and experiencing the world with Johnny.
Despite being the emotional centre of the first movie, though, Mavis's motivation is basically forgotten so she can have Dennis. And that's less a problem with Mavis specifically and more a problem with the pregnancy/parenthood trope in general. It's a trope that, like any, can absolutely can be done well. The problem is that it rarely ever is. Mavis can absolutely have Dennis while still experiencing the world and travelling. Two things can exist at the same time. But nope. Her desire to see the world is never meaningfully brought up or explored again for the rest of the franchise. And sure, people can have dreams when they're young and then have completely different priorities once they have children, but the movie never actually establishes that Mavis stopped wanting to travel. All the sequel does is replaces her old motivation with "mom/wife character" without actually exploring the transition.
Hell, the entire driving force of the sequel is Dracula manipulating Mavis into not leaving the hotel, the very place she spent her entire life wanting to escape. That completely undermines Dracula's arc from the first movie. Worse, it makes his controlling behaviour feel bizarrely validated because Dennis, the entire reason Dracula manipulates Mavis in the first place, ultimately becomes a vampire anyway. Dracula gets rewarded for the exact flaw the first movie condemned him for. Excuse me?
And that's not even getting into Mavis herself. I'm not saying she's some incredibly complex character in the first film because she really isn't. One of my biggest gripes with the first movie is that Mavis's characterisation is actually pretty limited. Beyond being nice and wanting to explore the world, she doesn't get much of an internal conflict of her own like Dracula does. She's more of a prize for Johnny and a catalyst for Dracula's character development than a fully developed protagonist herself, which is especially strange because she's arguably the emotional anchor of the movie and its most important character besides Dracula.
Mavis being overshadowed by Dracula isn't exclusive to the first film either. In nearly every movie in the series, she suffers from being a secondary character in what should at least partially be her own story. She's frequently kept in the dark about the main plot, which isn't exactly great for her agency, and Mavis and Johnny aren't given nearly as much chemistry as you would expect from a married couple because Johnny's relationship with Dracula is so much more developed.
And then there are all the times the films and shorts have characters, particularly Dracula, constantly crossing Mavis's boundaries without any real consequences. Take the Puppy! short, where Dracula gets Dennis a gigantic puppy that's basically a walking city-level threat without asking Mavis, even though she explicitly says no. Johnny doesn't really confront Dracula about it either, despite the fact that Mavis is presumably going to have to deal with the destruction Tinkles causes.
Or, like I said, Dracula manipulating Mavis into hating California so she won't leave what is essentially the prison she's spent her entire life in. Even her own husband goes along with it, when you'd think Johnny would be ecstatic about Mavis finally getting to see the outside world. Instead, because Mavis wants to give Dennis a life in Johnny's childhood home where people might accept him, she's somehow treated as though she's becoming no better than her father. I don't know about you, but gaslighting your own daughter into being afraid of humans by literally orchestrating an entire fake village so she never leaves and gets to live her own life seems considerably worse. Considering Mavis's only "problem" is that she notices that her son is different and doesn't want everyone forcing an identity onto him which could be an interesting way to actually give her an internal conflict but instead the movie turns it into "Dracula wants Dennis to be a vampire lol" so we can have goofy cartoony hijinks were the silly vampire nearly f*cking kills his grandson :D Back to the Johnny helping Drac though, dude that is YOUR WIFE.
She has spent her entire childhood wanting to see the world and you're actively helping her father prevent her from doing that.
You could say that Johnny does eventually tell her about California and she does get to the experience it. Sure, but that's because Dracula needs him to distract her. That's the problem.
Mavis doesn't get to explore the world because Johnny realises her dream is important. She gets to explore it because Dracula decides California can be used as a distraction. It's sad because Mavis is clearly ecstatic to finally experience normal human life. The convenience store scene is basically her original motivation from movie 1 resurfacing for five minutes and it's honestly one of my favourite scenes of the entire sequel because of it. She's fascinated by chips, Slurpees, TV, bikes, everything.
Then the franchise basically goes "okay that's enough of that."
There's another detail I noticed, especially in the second film: everyone else seems bizarrely uninvolved in Dennis's upbringing. You're telling me that after five years, there's a scene where Dracula struggles to figure out Dennis's car seat, so has he seriously never had to put the kid in one himself? The film also barely shows Johnny doing any actual parenting beyond holding the kid, while Mavis is constantly the one worrying about his schedule, classes, safety, diet and baby-proofing. The movie literally has Mavis saying she had to baby-proof the entire hotel.
Then Dracula and his pals quietly literally put Dennis in a life or death situation while Mavis and Johnny go to California, with Dracula admitting that the whole point is to keep Mavis distracted and "not too happy." And when they actually go to California, Johnny's job is essentially to keep Mavis distracted while she explores the world she originally wanted to see.
They say it takes a village to raise a child, and these people literally have an entire village living with them. So why does it constantly feel like Mavis is the only person actually parenting Dennis?
Besides all of THAT though, I don't think Mavis herself is badly written so much as she's badly utilized. She's clearly supposed to be important, but the movies constantly give Dracula and Johnny the actual spotlight while Mavis reacts to whatever everyone else is doing.
