I have camped at Diamond lake probably 20 times in the forest service campground. The campground is not the best unless you get into the "E" loop as the sites are large some with waterfront. They have a midge hatch the early part of June that people think are mosquitos and freak out over them. They are obnoxious but do not bite. All of Diamond Lake's shortcomings are nullified when you get on the water and start pulling in the big rainbows. Early morning, the lake is glass, Mt. Thielsen and Mt. Baily bordering each side with Crater Lake Rim to the South and a 3 pounder on the line. Doesn't get any better!
If you do go in the summer and it is hot hike down to Crater Lake and go for a swim. Swimming out from the shore and watching the bottom drop out from beneath you and the water change color to dark blue is something you will never forget. The water is cold and the hike out is rough but it is bucket list worthy.