The common expression was "The weather is beautiful. I wish you were here." The joke is switching those words. "The weather is here. I wish you were beautiful."
How would a girlfriend feel if she got a postcard saying "I wish you were beautiful"? It's just a play on words.
I regret introducing the phrase. I'm tempted to delete that post and every one dealing with it.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
***** NOT A TEACHER *****
(1) Let's say that Tom is visiting Paris. He might send a postcard of the Eiffel Tower to
his girlfriend in New York City. Usually, Tom would write on the card:
The weather is beautiful. I wish that you were here.
(2) But let's say that Tom wants to be funny. He might change the position of the
words. So he would write:
The weather is here. I wish that you were beautiful. [He is telling his girlfriend that
she is not beautiful!]
(3) When his girlfriend in the Big Apple (New York City) gets the postcard, she will
laugh and laugh and laugh. Some (many?) native speakers think that changing the
position of those words is really funny!
Until I read Barb's post, I had never heard of that joke. (I like only pop music of the 1960's.)