The adjective's fine. It's supposed to be used in the same sense you'd use "serial killer" or "serial drinker," as in, someone who continually engages in some vice. It's a bit tongue-in-cheek, since there's really nothing wrong with starting companies, which makes it even more enjoyable to use.

I do oppose its misuse, however. PG's clearly not one, having only done one startup. These guys actually are, though.