Is she an evil fairy?
No. She is just grim, but not evil.
Then how come her transformations look so different from the show?
Because the transformations are adapted to the character, not the character to the transformations. I'm not going to change Ombra's grim appearance just because it would fit to the transformation. Besides, I do not like the appearance of some transformations - Believix is too sweet for me, and in Butterflix all fairies look the same - that's why I'm change these transformations in my way.