It does rain in deserts, California isn't mostly desert (about 38% by land area), and drought is defined relative to normal rainfall, so even a place that usually has very low rainfall can have droughts.

I don't get it.

It does rain in deserts, of course. But most of California is not a desert anyway.

And also droughts are defined by lower than normal precipitation. So if it didn't rain in a desert, and it's still not raining in a desert, that wouldn't even be a drought anyway.

It’s a joke relying on several layers of not understanding basic concepts for its humor. Kind of worried about the environment in which it “hit well”.

It must only be funny to people who are unfamiliar with California’s climate.

Are you under the impression that California as a whole is a desert?