Gosh Francine looks so weird Flash-drawn/animated.
But as for Jenna, she seems to be one of the most-hated characters from the Arthur universe, mostly because of her overall design; granted, she's not the nicest-looking character from the series, but I don't think she's "so damn ugly" either.
My problem with Jenna is she really doesn't seem to have too much personality. Her shining episode, "Jenna's Bedtime Blues" did shed some light on a somewhat touchy issue with children (admittedly, I mentioned on Muppet Central several years ago I thought it'd make an interesting, though probably controversial, story that Ernie "has an accident", but doesn't want Bert to know for fear he'll make fun of him... and this was before that Potty Time DVD, BTW), but other than that, she's like another Francine: tomboyish, a sports junky, etc. I think if the writers tried something a little different with her, she might have been a more interesting character, but other than that, with as many different characters, both regular, or recurring otherwise, as the series has, it's kind of hard to explore any other kind of character with him/her being too similar to another already-existing character... which may actually explain why it seems like almost all newly-introduced characters have some kind of disability or disorder that is focused on them.