The only really unlikable character, IMHO, is Marigold. I've tried to warm up to her, but she just has no redeeming qualities. While I can relate to portions of her personality (I was a socially awkward dork at one point, as I'm sure many posters have been) that she still acts like she's in high school in her early-to-mid 20s is grating. Plus she's seemingly dull, lacking the intellectual heft that the nerdy often have to counterbalance poor social skills. Her pairing with Dale still seems inexplicable to me, because besides his lack of experience with women, Dale seems to be one of the most responsible and together characters in the entire strip.
She's someone I'd take in small doses, certainly not someone I'd have dated. But I don't know that I'd go so far as to say "unlikable," that she lacks intellectual heft, or that she has no redeeming qualities. Jeph himself has said that her "self-esteem [is] undetectable with current technology." I think -- basing this off people I've known -- that she's probably book smart, but needs a bit of work in the common sense and social graces departments. What little we know of her backstory suggests that she was bullied; her mom's never been mentioned, and her dad seems to be as socially stunted as she is (though I could, admittedly, be reading a bit much into it; we haven't seen much of him); and, combining those things with her insecurity, it's entirely possible that these are pretty much the first friends she's had in a long time, if ever. So some of the perceived immaturity could be coming from a simple lack of socialization that came about, at least in part, as a defense mechanism -- when people aren't just an annoyance but a perceived or potential danger, you're more likely to keep them at arm's length. We've seen her start to open up, and we've also seen times (like when she attempted to take care of Hanners around the start of the wedding storyline) where she's tried to be a good, or at least better, friend to the people around her. She's got some growing up to do, but it does look as though she's trying.
Regarding her relationship with Dale: I don't see how that lasts. It's like any other relationship -- if you haven't got anything in common beyond sex and a few pop culture touchstones, you don't have much of a foundation. I don't think that anime's enough to hold a relationship together by itself. Either Dale backs out because he's got his act together more than Marigold, or Marigold finds enough self-confidence that she realizes that she and Dale have very little in common and decides to find something/someone else. It's also worth mentioning that Dale/Marigold and Faye/Angus developed in much the same way (guy somewhat quixotically pursues disinterested girl, gets girl); what remains to be seen is how the Dale/Marigold endgame ends up different from Faye and Angus's.
The only reason I'm not sure about the "Marigold runs off to NYC with Angus" angle is that Marigold was, for a time (not sure if she still is) Jeph's favorite character. Then again, Hanners was once, too, and we hardly ever see her anymore... so maybe you've got something there.