Looking way too far into things but as Michael Richards took the Kramer character with great thought, being a seasoned actor, I don't think this is taking things too seriously.
He started off wearing bathrobes and supposedly rarely left the apartment building. His hair was also "down". This is again seen in the backwards episode where 11 years in the past (the show has been running for 9 years at this point, so the scene takes place 2 years before the pilot). His hair is down here also.
In The Airport, Kramer flattens his hair when trying to get John Grossbard who he hasn't seen in 20 years, to recognize him.
When Kramer begins accessing the outside world more frequently again, his trademark vintage clothes begin showing up. Richards says this is because "these are the clothes he's always worn, and never bought new ones". Despite this, Jerry says Kramer always wore jeans when he first met him (Kramer is known for wearing slacks, usually too short for him), even though when Jerry actually first meets Kramer, he just lazes around in bathrobes and such. We also learn a few times throughout the series that Kramer frequents the vintage clothes store Rudy's often (which would explain how his wardrobe is always growing). Richards said Kramer had around 46 shirts in an old article I read on his costume a while back.
Kramer states in "The Strike" he has been on strike for 12 years, meaning he had stopped working one year before Jerry met him (The Strike takes place in S9).
So here's my theory using things we know about the character.
Kramer had spent a long time travelling and likely lived with Grossbard when he was in his late teens (remember he left home on a steamer and went to Sweden when he was 17, stated in "The Letter", so maybe he ended up staying with Grossbard). He had also lived in Greenwich Village for sometime, this may be after; and then lived in Los Angeles sometime after that for a few months. I assume when he moved to New York, he got his job at H&H bagels to support himself. On subject of outfit, he would have been picking up clothes at second hand stores throughout his life ro save money; the retro shirts would have been pretty trendy when he was a teen and he has simply kept them, probably the same for the pants and him growing up would explain why they are a tad short (quote Richards saying that they are the clothes he has always worn).
He went on strike from H&H and then didn't have to work, turning him into a guy who just lazes around all day (think Big Lebowski). He was probably getting money in from other things; Kramer states he has written a book "Astonishing Tales of The Sea", so he could in fact be making money from that, but not stating it. When Jerry meets Kramer, it is in his "laze around" days. On the subject of jeans, maybe Kramer tried to look like Jerry in admiration, or was simply going through a phase where he thought he would try something new or an attempt to go mainstream ("I was a different man then, Jerry!"), before going back to the usual choice of clothes.
Kramer's "get-rich-quick" schemes slowly begin at the start of the series, implying he is now running short on cash from going on strike from his job about 2 years ago (from the start of the series). When he finally decides to enter the outside world again, he slowly removes himself from lazing about in t-shirts and whatever and begins dressing in his trademark way. His hair was him trying to spruce himself up a bit also.
That was a long essay, but what do you all think of this theory?