It really depends on what kind of library you’re at. University libraries: you may not find a spot to sit if you’re at the wrong time, hell I know a professor who hangs out at a different university’s library reading and writing; major city public libraries: there’s probably a few people doing so and it isn’t weird; small town/suburb/small urban branch public libraries: it’s very local culture dependent, but you’ll definitely find a decent chunk of them seeing it as strange