I know it's not close to Shotstic's first answer, which the author said was close to the actual answer, but here we go anyway.
I think you may be..
an apple.
*I am a source of good ideas*
*But I am also a source of great tragedy*
>!In Genesis 2:16-17, God forbids Adam and Eve to eat from the tree of the knowledge of good and evil. Generally the fruit of this tree is portrayed as an apple.
Witches always find it funny to curse me
In multiple fairy tales witches poison or curse apples so that when the victim takes a bite, they suffer some sort of a curse.
Creeps and thieves never seem to like my presence
But only if others are around with me
In medieval times, people would throw rotten food at pilloried thieves as a form of humiliation. I suppose creeps could have also suffered the same fate. Apples would have been a common choice of rotten fruit to toss.
Most important of all, I am the mirror of a question
I think this is a red herring. "Mirror" in this context could mean opposite of, or response to. The response to a question is an answer. But answer to what question? Well what question follows on the following line? "What am I?" So I believe this is a red herring which basically says that I am the answer to the question "What am I?" Also, most apples are red like this red herring. :P