Dunno about that. The story centered around Mario & Luigi's relationship as brothers was a good call, and Mario feeling personally responsible for his brother's well-being is what carries him to do such wreckless things to begin with. Mario never wanted to be a hero, and even the entire cast seems to treat his presence as kind of a third wheel. I honestly that felt like a refreshing take on the typical "savior from another world" template.
I just feel people are too cynical to take it as anything other than wholesome. What's Ironic is people complaining why Peach is "such a girlboss" in this one, when the same people complained how the Mario cliche of saving the princess has been done to death.
So yeah, it's not the best movie out there, but is it bad? I can't really say it's bad at all. If you want bad, try the 1993 Mario Movie.