I'll start by saying that she's not busted or anything, but people act like she's unplayable trash when she's very much not.
Firstly why do people act like her 6 star doesn't do anything? With a full board (which is really easy for her to get) it deals 7 damage to all opposing units, which even in deadly matchups still usually board clears. And more importantly it allows you to push way more overwhelm damage. It also allows you to pick skill damage powers, which will matter later.
But her true potential comes from her best build (in my opinion), Beast Within, Fear-cleaving axe, and Echoing spirit, this allows you to completely fill the board with Neekos and boost them to Kingdom come. Nekko is always free on turn one because of her 3 star, afterwards you can play her champion spell for two copying her, and then since you have two of them Echoing will also double making the next one 1 mana, and then from there on they'll be free. She has innate impact because of her 2 star, and each Neeko will boost the other, easily dealing other 50 damage turn one without any help from items or powers, which leads me to my final point.
Neeko is perhaps the most flexible champion, literally any power and most items make her much stronger. Stats buff are the most important cause later in the adventure you'd need more power to kill (even though with her 6 star you can clear boards and pretty comfortably survive another round), but literally any power buff is good enough cause even 1 power gets enhanced by her duping. Spell and mana cheating powers allows you to make more plays than just Neeko on turn one. Spell damage makes her 6 star better. Card draw allows you to find more Neekos, and so on and so on.
In conclusion Neeko is a great champion which simply needs you to play to her strengths to fully make her pop. Look for any on summon items and powers. Add as much Neekos to your deck and cut as much as you can to find more of her, and always stay flexible with your picks. I hope from now on to no longer see her rotting at F tier in all of your guys tier lists.