Mitsuri is said to always be rather annoying by Shinobu who said that she was also the most disliked Hashira due to her bratty-like attitude. She is also said to throw tantrums when she does not get what she wants and makes a big fuss over such little things.
But, overall, Mitsuri isn't a horrible person as her being a Hashira meant she has saved plenty before, something that she would gloat about yet still be proud of this achievement as she made people's lives less miserable. 