Why does Marty's underwear have the name Calvin Klein on them

because it the brand of underwear

It is Calvin Klein because that is just the brand of u underwear that he wears and it fits in perfect for Lorraine to think that is his name because while he is unconscious she takes his trousers off and that is what his underwear says.

That's the brand of underwear he is wearing.

Doc used to wear the polka dot ones he gave them to marty