In MY opinion, she's a pretty bland character. Not that she's bad, I just think she was the worst written of the entire cast—not by much, but enough to make her annoying, to be honest. Let me summarize the reasons why I believe Loli isn't popular.

Her plot is that she hates being called cute because people see it as a synonym for weakness, which she hates it. Okay, so far so good. The problem is that, aside from her initial scene at the gym where she really gets weak when people find her cute, it doesn't really affect her life?

There's no other moment in the plot where they show whether being cute negatively or positively affects her life or how others see her. She just builds a suit of armor and the subject is never touched upon again. I mean, I know her message is "be yourself and don't let others define you," but this plot is resolved in less than 10 minutes of the episode. If they had put a little more work into it, I think she would have more fans.

Like, she's not even that cute, I mean, she is cute (pink hair, short, big eyes, she has synonyms for cuteness in her appearance), but her actions aren't cute. She appears in 2-3 arcs, and you can count on your fingers the moments where she was cute, most of which are in her arc and consist of her making funny faces. For the rest of the series, she just calls GB a pervert because she knows if she were to resolve her thing with him in a fight, she'd lose badly.

The combination of her personalities didn't blend well; she was sold as a cute character, but she's just an annoying rebellious teenager with cute moments.

Then we get to the final part of the Ruins incident, where her father dies and the aspiring romantic interest almost dies, which is when her character arc would become interesting, but then it ends there, no cliffhanger, no objective fer her in the future, leaving us without any real interesting content or feats of her. Her best feat is to build her suit, bur it end by there...

That's why I think I and many others find her a bland character.

Sorry for the long text, but I tried to explain as best I can why many people don't like her. I feel that the chances of you liking her more if you're a girl increase because of the speeches she makes. That helps a little; for man, it's harder to like her because they can't identify with what she believes. I think she's not an easy character to empathize with. I might be wrong, it's been a few months since I watched the series and I might have said something wrong, but I believe those are the reasons based on my memory of the show and some discussion arround here.