Mystra is not an evil goddess. The incarnation of her, that we meet in the game, is pretty young for god standards but Mystra scene s important to keep magic in balance. Last time she died, it was chaotic.
She is not really a character in the game, a player would necessarily root for, but if Gale chooses to just leave the crown in the Chiontar, his orb is still stable ( or healed even, not sure right now, but he lives a normal life), so she isn't really petty either.
I don't have strong feelings about her or most other gods in the game apart from our local resurrecter, Jergal., who is good in my book. I think the dead three are pretty stupid, but then, they were basically bullies back in the day.
Mystra should have had more faith in Gale, because if he would do, what she wants in act 2, it would make the situation worse. But I see that as the gods not knowing everything and she is forced to act indirectly by Ao, so she does, what she thinks is best. And again, she isn't petty about that, if Gale acts differently.