[geeman]

But as far as being mandatory,
if you mean as in 'you can't go in without one', the answer is no. If your mom feels best in shorts and a t-shirt, that's perfectly fine. She just won't be able to go into the pool areas or slides and such. But there are plenty of spots at the waterparks for people who don't swim to just sit aside in the shade, or get some sun, or even just relax.
My 80 year old dad once came with me and he barely rolled up his pant legs just to walk on the sandy bits. He enjoyed just being there watching everyone have fun.
And I also agree with the other posters- this is a place to go and have fun - most people are so involved with the "fun" that they don't even notice anyone around them. So tell her not to worry. :-)