I don't think requiring masks for everyone is a violation of the ADA. If you have a service dog, but the dog begins barking, lunging, or defecating in public, a business has the right to refuse service until that dog is removed. Accommodations for access end at the point when they become a risk to the health and safety of everyone else.
We are in such a situation right now. If the mask was only effecting the person wearing it, accommodations are possible. But masks in crowded, unnecessary places like theme parks are necessary for the health and safety of everyone, including the employees.
I am actually in Florida and I wore a mask to go to a park the other day. They recently opened and you are required to have a mask on your person at all times, and to wear a mask when you are within 6 feet of another person. I chose to wear the mask the whole time because I felt it was risky to take it on and off repeatedly, and also because on trails people were biking or running close to me, I had to cross a bridge with people fishing, etc. I was fine, this was an 88-90 degree afternoon in full sun, no AC breaks. I walked the dog for about an hour and a half, lived to tell the tale. It was great to get outside for awhile, all of our natural areas have been closed since March.