It’s hard to say exactly what went wrong with The New Mutants because it doesn't succeed on so many levels. Those who follow the X-Men franchise will likely be let down by the characters' abilities, and viewers looking for a suspenseful story won’t find satisfaction in the antagonist. I would only suggest seeing this if you're a big fan of one of the performers; even then, there's a chance you might feel underwhelmed. If I were to offer suggestions on how to improve the film, they would be: first, replace the villain and move away from trying to blend it with a strange horror story—instead, concentrate on what made X-Men appealing in the first place; it needs a real antagonist. Second, there’s too much emphasis on the weak backstories of the main characters. Third, the powers should be more impressive! They ought to make viewers wish they could have them themselves. Finally, remove the corny lines and decide on a specific genre—it feels like a mix of science fiction, action, horror, romance, and a low-budget movie.

Source on TMDB