Sleepover may have killed a Janna/Marco romance, but I think those who say it also killed any sort of Janna/Marco relationship are going way too far.
The possibility of them being childhood friends is still very much alive. It can't "just" be about picking on Marco for being an easy target, for Janna shows genuine concern over Marco's thoughts and opinions: she got upset when Marco didn't want to play Truth and Punishment, she got excited at the idea of Marco liking "blood red," and she looked genuinely upset when Marco slapped her down so easily and even callously.
I'd also point out that Janna answering that her crush is on a deceased poet is a potential sign of immaturity with regards to romantic interest. It is possible that she isn't quite full into the "liking boys" stage of her life yet like Jackie or Star. If so, then there's always potential for change. (But maybe I'm just deluded by how much I think Janna would be a far more interesting rival for Star than Jackie, especially as Janna's is moving up to a position as the third party in a Star-Marco-Janna trio while Jackie remains far removed from Star and Marco.)