I just saw the film and well... I can see why critics don't like this film.
I don't think this film has much to offer to anyone who isn't a huge Mario fan. I feel like things happen in the film and they are never explained because the film makers expect everyone to already know the lore of the games. I didn't feel like there was any build up or atmosphere or "Chekhov's gun", things happen because [insert Mario reference]. And I guess if you like seeing Mario references, then this is the movie for you.
I did like pieces of this film. Bowser was funny, I like what they did with Peach, and the animation was nice. Tho I felt like the movie had pacing issues and I wasn't a fan of the dialogue.
I will give it a 5/10. If you like Mario, it's worth checking out, kids might like it.