I've heard some fans find some kind of attraction in Francine, though then again, that was a number of years ago on an IMDb board, and admittedly, IMDb is full of crazy people, lol.
Now, I'd argue that there's a little cuteness to Fern, but I think moreso with her in earlier episodes, when she's more shy and mousy... the cute streak kind of disappears as she becomes more outgoing... and especially when you upset her, and then she begins to show something of a passive aggressive side.
And of course, D.W. is a natural at mastering that, "Look at me, I'm such a cute lil' girl" front to get what she wants... I especially love how Arthur used that to his advantage in "Arthur's Snow Biz", Buster even saw past that scheme of his, "Getting your little sister to find you work? No fair! Who's going to say no to a four-year-old girl?" "No one!"