I guess that's what happens when the directors (Annabel Jankel and Rocky Morton) do a Mario movie where they have no knowledge about the history of the Super Mario Bros. game, like we do.
Like the goombas, for example, they've should've been like the little grumpy mushrooms that we know, not some overgrown 8-feet tall lizards. Plus, Bowser (or Koopa as he known in the movie), he's supposed to be the terrifying evil turtle-like reptile (or King of the Koops, if you will) with his famous spiked shell, not a human-like tyrannical politician or something.
Sorry if those sounded like spoilers, but that's basically some of the details that were considered wrong for this one, thus resulting in the flop that was made.
Overall, the details of the movie were way off. They could've made it like, i don't know... in marionation or something, kinda like Thunderbirds (for some of the familiar Mario enemies that we know).