It may not be the best time for lying around the pool, or visiting the waterparks, but it's definitely the best time for touring the parks. As many others have posted, the best plan is to "layer."
It's going to be cool in the morning, so you'll need a jacket or sweater. Then it will warm up, then get cool again when the sun sets. I've found that I'm comfortable with a short-sleeved top, long pants, and a light jacket or sweater that I can fit into my tote during the afternoon. I've also added a sweater covered by a windbreaker if I'm going to watch Illuminations.