Alicornutopia

You better believe she did. :)

At some point in the distant past, there was a Matilda in a Sunny world who lost. Since she never got a chance to go around being highly contagious at the universe's magic, it was dominated by her Truchbull-equivalent, who used it for lightning bolts and kidnappings. Its personality snowballed from there. And that's why Sunshine-type universes got in the habit of being really unfriendly with their magic.

I’m fairly sure the standard explanation is “all the evil demons did it”. I suppose evil magic could have caused the existence of evil demons in the first place.

I think he meant an Demon!Mathilda or Ancient-Past!Mathilda plus Truchbull-equivalent?

I didn't think it through that much! But I'm pretty sure it's compatible with both.