My son never cared about dressing up at all, and he overheats easily, so it was just as well. My daughter absolutely loves to dress up and would wear princess dresses all day every day if I let her. When we've been at DL, WDW, and on Disney cruises in the last year and a bit, she's basically lived in costumes (when she isn't in a swimsuit). She would be miserable running into other kids in costume if she weren't in costume, since dressing up is one of her favorite things to do. We plan around it. On cooler days she wears her longer sleeved warmer costumes, on warmer days we go with light weight and short sleeves. If she'll be mostly inside in climate controlled areas, she can wear whatever she wants.
That said, I would not plan something likely to be hot, like an Ewok, in the late spring/early summer, except to wear to a specific meet and greet or character meal. If you think you've got a kid who will really enjoy being in costume all day, try to make sure he or she is in something that won't be too likely to encourage overheating. I might do some pictures in the Ewok outfit, then have a second costume to change into.
