The jacket just looks like a variant of “morning dress”. It’s the equivalent of a tuxedo for the daytime (wearing either at the wrong time of day used to be considered incorrect; see the selectively-sartorially-fastidious Jack in 30 Rock reacting to Liz’s surprise at his wearing a dinner jacket without some special event planned with, “it’s after 6:00, Liz, what am I, a farmer?”)

You still occasionally see them at state functions (Trump wore an infamously poor-looking one when visiting Queen Elizabeth in his first term, iirc, and you can find photos of people like Reagan in it looking a bit less uncomfortable). I think they were standard/required clothing for arguing in front of the Supreme Court through the 1970s or something.

It’s the kind of jacket one might imagine a stereotypical cartoon mayor of a town wearing for a daytime ribbon cutting… because, not that long ago, that’s exactly what they would have worn.

It’s an almost, but not quite, dead piece of clothing, but it was still quite familiar when this was made.