We've barely seen 10 mins. of Mari. It's kind of hard to determine a personality in such a short time, and even then we saw some glimpses of it (she commented to Shinji how she also had doubts about piloting Eva despite having seen her so into it in the other scenes). I don't get the Mari-hate.

And tbh, almost all the characters in the movie seem to have "no personality" if you haven't seen the series. So, it's obvious that Mari would be highly underdeveloped in contrast with the others.