I remember reading a comment about this when it was posted elsewhere a while back. They said at the time that the mask was actually not as creepy as we think it is now. We have generations of people that have grown up watching horror movies and playing video games like FNAF that have altered our perception of creepy. In the same way clowns used to not be creepy. The comment also mentioned that since Mickey was a beloved icon for children it actually comforted them to wear a mask in that form. Also it's worth noting that there were much more horrific things occurring daily for them so this was probably quite tame in comparison.
This comes from someone else who heard about it from their grandparents so take it with a grain of salt, but it wasn't necessarily a scary thing for them at the time.
