"I take off my clothes in the summer."
It can be. The difference here is that by using は, you are introducing summer as a general topic (& leaving context to make clear that it is the time of the action), wheras when using に, you are not introducing a new topic, but make it explicit that summer is the time ("temporal location") of the activity.
は is the for topic of a sentence. If we said このねつはさむいですね、then we'd be speaking about summer, but we are not. We are speaking about ourselves and our actions. I'll double check with my Japanese teacher, but I'm almost certain this grammar is incorrect. You wouldn't say 明日は本を読みます。you'd write 明日本を読みます。and I'm sure this the same rules apply here.