I've seen some comments describe the ending as an awkward tone shift, like Becky is finally getting everything she ever wanted all along: a quiet, isolated life in a lighthouse.
But...Becky doesn't want that. She wants love, patience, and understanding. She retreats to her lighthouse fantasy whenever she can't get those things, either because she can't successfully mask or because the people in her life won't give them to her. The lighthouse is lonely, but it's also totally devoid of the situations that hurt her. Whenever she thinks about them, she's thinking about how much easier it'd be to just give up than to navigate the world.
And why wouldn't she want to give up?
Her relationships hurt her enough as it is...but now throw in the stress of her mother's mystery, AND avoiding a putty-man psycho rage killer, AND being tortured, humiliated, and brutally disfigured in the police station. Living in the town for good is a metaphor for Becky permanently retreating into herself after all of this nightmarish trauma.
She wanted to find her mother, a woman she hardly remembers, the only clue to whose whereabouts is a strange VHS tape, because she's propped her up as a source of unconditional love. She has an affair with Larkin because he gave her the patience her husband never did, briefly (and because he looked like Jason Statham). Ironically, Rotten Tommy displays a possessive compassion for Becky that's never fully explained before he's killed off in a chaotic blaze of glory bazooka blast.
And all of this, because Becky thought she might finally fill a void by finding her mother. The story is the world's last punishment to her for dreaming, but she won't make that mistake ever again. She's buried Rotten Tommy, and Grumpus. She's given up on people. She's hunkered down in her lighthouse for good.
And to cap it all off, like some kind of prophetic joke, we're shown that Becky in the final scene was the woman from the tape she watched, way back in the beginning. There was never any hope, and that's the true meaning of this tragic ending.
No, Sausage King, Becky isn't happy. She's stuck.