Some people just don't like small furry creatures, that's what I've found. A good example is a friend of mine who absolutely hates my gliders (no offense) but loves her horses. She just likes big animals, and small ones are gross to her. I take no offense, that's just how some people feel. So, maybe your mom just doesn't like tiny mice-like creatures.
The fact that your mom fed them and was willing to hold one even though it bothered her is very sweet, and shows how much she cares. I think that effort is worth noting.