I'd say this movie is roughly a 6.5/10. I don't really have a whole lot of negative things to say about the movie, and for a video game adaptation, I'm going to say, it isn't completely awful. That's said, this movie really needs some work.
Firstly, the characters are just- like they don't feel like they have a lot of depth. Like Luigi just get sidelined most the movie and doesn't get time to shine, but he's introduced as a super important supporting character and then he becomes a bench warmer character most the movie. I mean, he gets time to shine, but it's mostly in the shadow of Mario.
Another character that doesn't get much depth is Toad. Like, he's just there to glaze Mario and have cringe worthy lines. I mean, he gave princess peach an ice flower? That was important, but- you see where I'm going with this now? A lot of characters are given roles that just aren't utilized until the very last moment, so it feels inorganic and the cogs don't quite mesh.
Secondly, a lot of the dialogue is just really corny or basic or well, just not fun and super rigid. Like, I get it's a kids movie, but please, don't give lines that are icky. You don't have to do that just because it's a kids movie. A lot of Toad's dialogue was just an ick for me. Same with Lumalee. You only got the Facebook mom's laughing with that character's dialogue 😬
Thirdly, please, illumination, for the love of God, I know you use pop songs in every movie you use, but this was not the movie to do that. And if you were going to do that, you could've used any other song than "Take on me" during the Donkey Kong segment. I was visibly confused upon my first watch when I heard this song. Don't just shove songs into your movies. Don't shoehorn things in, please, if you are going to use a pop song for the sake of getting ears to recognize it, use one that makes sense.
Now, aside from that, this was a relatively watchable movie with really good pacing, a likable lead with (mostly) likable side characters (even in spite of the narrative flaws) and was generally and not terrible experience. One of illuminations better movies, one of the few that isn't an utter travesty, so honestly, I don't want to gripe about any more flaws I end up picking up later and will just rake the scraps of goodness that I can get from illumination, because it will be a while before they release another somewhat decent film.