Perhaps he meant it in an affectionate way?
One wonders why he appeared in a movie based on a comic book villain if he thinks that way. Oh yeah, because such movies are popular as Fxxx and it would give him a better profile. I'd never heard of Marc Maron before this. Ah, here we go, it was an opportunity for him to work with Robert DeNiro. What a laugh.
I suspect Joker will be very successful but Marc Maron will be consigned to further obscurity after it is all wrapped up.
I sympathize with the statement. "Grown male nerd childs" are the primary audience for superhero movies, but not the only one.
As far as why he'd be in one--actors take jobs when offered and (usually) do their best. They don't have to like them, as long as the check clears.
I'd say Marc Maron is pretty well known, though--not a star but definitely well known. Per wikipedia his podcast was downloaded 100 million times by 2013, who knows what it's at now.