Hadn't been to Lake Thunderbird since I was a kid (40+ years ago) and all I could remember is that we always called it Lake Dirtybird because of the water. As far as that is concerned nothing has really changed. Typical central Oklahoma red water. 

We stayed a couple of nights at Little Sandy Campground. There weren't any full hookups and we had to stay at a spot that was water and electric only ... and boy were we glad. The full hookup sites in the next campground over -- Hickory Hill -- were packed in like sardines. Our site on the upper loop of Little Sandy was away from all the hubub but also away from the water, so there was that. If you don't mind being right up against the next camper, Hickory Hill has some full hookup sites right on the water. However, the upper loop of Little Sandy was nice with all the trees and the better-spaced sites. In my opinion spot 36 is the best one. No one can camp behind you and there's a lot of space to the south. The dump site it up on the main road about a quarter of mile north from the campground entrance and easily accessible.
We stayed the first weekend of May 2020 ... the first weekend some of the COVID-19 restrictions were lifted. The place was PACKED, especially with tent campers. No judgment, just saying there were a lot of them. It was tough finding a path to the water without walking through someone's campsite. The water pressure was pretty low at the site and my guess it was due to all the sites being in use.
All in all it was a nice weekend, but I've stayed at nicer and quieter places.