We know that Yukiho has redeeming qualities and we know that the story has a happy ending (in at least the immediate future) because the structure of the story is a light-hearted RomCom, not an angsty drama. The mangaka didn't telegraph that love confession in front of the fountain for nothing.
Aside from which, the omake makes it even more clear. They've been dating for a month (wouldn't happen if Yukiho didn't have redeeming qualities) AND Yukiho confesses love to someone for the first time. She's a good egg even if we don't see it on the page.
I'd even argue that in certain sense we do see it on the page. In their daily interactions Yukiho is, in fact, very nice to Meguru, and they do have fun together.
What makes her seem so awful is that she gets the narrative treatment that manga two-faced villains always get--they come off as super-nice in person, but then later when they're in private the reader learns that they have some ulterior motive for manipulating the innocent victim. But the worst thing Yukiho actually does to Meguru is think to herself how immature the younger girl is while being nice to her on the surface.
So Yukiho seems to have that hidden contempt for Meguru that the manga villains do, but she's really just enacting that facade of niceness that she's learned as a life-coping strategy. She doesn't try to humiliate Meguru or steer her wrong, and after she pushes Meguru away she realizes that, although she initially just wanted to use Meguru "to fill in her free time," she later really wanted Meguru to get close to her.
Yukiho is basically a bit of twist on the trope of the two-faced villain who is nice to the protagonist in public but silently sneers at them in private--but in this case it's due to her own low self-esteem and poor self-image rather than because of any ulterior motive or bad intent.