Jim mocks Sargon's pseudointellectualism, his Capos, his self-importance, and, most importantly, his shitty suit.
Jim has simply pointed out what Spencer pointed out, and a lot of other have as well. Carl isn't as smart as he think he is, or as his capo's, or his cult of personality think he is. He is just an e-celeb on the internet, that's it.
The whole suit thing is just funny, like really funny to me. A nice suit tailored from a tailor in Swindon would probably cost no more than 600 pounds, which lets face it with what Carl rakes in shouldn't be an impossible number to come up with. Hell even a good quality 2nd hand suit can be had from a charity store for about 40.
That being said, the fact that his whole suit thing basically made it look like he clearly doesn't know how to dress himself, even though there have been a few public appearances where he doesn't look like a basement dweller, and I really can't imagine why it didn't occur to him to not show up in a suit that looks like his mum bought it for him from Primark.
Knowing his exceptional arguments, I'd bet you he actually did it by choice, I don't buy the I bought it the night before story, he knew it wasn't a good suit and he didn't care, because it would allow him to play it off as an I'm a man of the people schtick he loves.
He probably just didn't imagine it would become a point of interest for Jim, who as a disenfranchised potato muncher, couldn't own the land in the old country, and couldn't possibly own a suit in the new one.