(posting this response because of Sam's comment about how nobody here likes movies).
Yes it was cartoonish, but I think that added some fun to what could have been a really bleak movie. Didn't read McDormand kicking high school students in the crotch as being "badass" - it's supposed to be amusing and make her seem like kooky fun, but take a sinister resonance by the end of the film when you've realized that she's just as violent as anybody else.
I should note that, while Billboards didn't remind me of Wes Anderson, Fantatic Mr. Fox is basically a crime film and is terrific.
What didn't you like about the ending? I thought it subverted the standard expectations of Rockwell's character bringing the killer to justice and getting a redemption arc in an interesting way. (This has been a frequently debated point about this movie, but I don't think we're meant to see him as actually a good person at the end).
I think the execution is definitely not perfect, but overall I found it interesting. And Rockwell and especially McDormand do carry it well.