I think you might be
Light
I am a source of good ideas:
Light bulbs in cartoons, "bright" ideas etc.
But I am also a source of great tragedy:
Blinding light(literally), UV light causing cancer, light can do a lot of damage.
Witches always find it funny to curse me:
I'm not sure if witches curse light, I can imagine something like that happening but I'm not an expert on witches either so I wouldn't know.
Creeps and thieves never seem to like my presence:
They usually like to lurk/operate in the dark
But only if others are around with me:
If there aren't any people around the light doesn't actually matter.
Most important of all, I am the mirror of a question:
I'm totally stuck on this though, can't think of anything.
Second try:
You're a (light) torch
I am a source of good ideas:
same as before? Bright ideas
But I am also a source of great tragedy:
torches cause fire, fire causes destruction/death
Witches always find it funny to curse me:
They are burned at the stake
Creeps and thieves never seem to like my presence:
Same as before, exposure
But only if others are around with me:
You can only be exposed when people are around. This could also point at people(carrying torches) chasing(or anything else really) the creeps/thieves.
Most important of all, I am the mirror of a question:
Again, I don't know what this means. Mirrors+torches were used to direct light but I can't figure out the question part. Then again, I'm not a native English speaker and might be looking at it the wrong way.