I think you are...
A light bulb
Because...
I am a source of great ideas
When the light bulb comes on (over one's head)
But I am also a source of great tragedy
When the bulb burns out bad things can happen
Witches always find it funny to curse me
I'm lost on this one unless maybe witch/light bulb jokes
Creeps and thieves never seem to like my presence
Obviously they like the cover of darkness
But only if others are around with me
Christmas tree lights mean Christmas presents to steal? (Would go with the previous clue.)
Most important of all, I am the mirror of a question
Think mirror image of a question mark.