While I'll admit our Alaska cruises weren't on
DCL, the weather is the same, regardless of what ship you're on. We've cruised Alaska at the beginning of June and the middle/end of September. The weather on both cruises was similar - mid to high 60s most days, drizzly rain in Juneau, and cool winds in Ketchikan. The biggest difference was there was a lot more daylight in June than September.
That being said, I will echo the fact that weather in Alaska is highly variable. I believe the comment is "if you don't like the weather, wait 15 minutes".