The truth about this movie is that it really isn't all that bad, it's just that this movie has so little to do with Super Mario Bros. other than the characters' names, it really isn't even a Super Mario Bros. Movie. It's just a movie that has Super Mario Bros. in the title.
Take Toad, in this movie. In the game, he was a cute little mushroom dude, in the movie, he's some weird guy who plays the guitar on the street and sings songs. And Bowser is not a menacing turtle-creature in this movie, he's a dude with funky ridges on his head.
This movie has an interesting plot, but no one will notice because they're too busy trying to figure out where the SMB stuff is at. Where are the funny-looking mushroom guys? The turtles with wings? Is that Yoshi? Why doesn't Mario ride him while he's slurping up enemies? Where are the perilous lava pits and the green mushrooms that give you extra lives? And where the hell is Kuribo's Shoe?
The movie would have been better off if they had just done away with the SMB license and released it as an original movie. So, if you want, you can see it, but try to imagine that it's not a Super Mario Bros. movie.