You think so? I can't help but feel that's not actually the case. If it really was about him being unable to write sympathetic characters, then his characters would not be engaging as they are.
Whenever I read Hamita what keeps me interested isn't just that the characters are weird, is that they are always wrong in just the right way to be unsettling. I actually think Hamita has the gift of storytelling exactly because of how weird his characters are. Their weirdness is what makes them compelling.
I have absolutely read mangas (and even books or movies) where characters don't feel real at all, despite them following tropes and largely acting how you'd expect them to.
Hamita is doing the exact opposite. He always makes them quirky, yet you find yourself caring for them and wanting to understand them. What he's doing is actually a lot harder than making a normal person sympathetic. He is making the alien understandable.
Reading the alien girl at first you really find yourself believing she might just be a little weird, but at the core has our same beliefs. She feels alive despite being utterly alien. And the more messed up stuff she does, the more you find yourself divided between disgust and hope she actually does have something going on for her. Even more telling might be the childhood friend in love with the main character. She was perfectly normal, there was nothing weird about her and she (just like the other friend in love with the gyaru) were very relatable as the only normal people in that manga. But then when she's remade she is utterly wrong. To write that radical change and make it feel so visceral you need to understand people very well and have a real gift for storytelling.
I'm not saying he always hits the mark, and sometimes things do feel weird for weirdness' sake, but I don't think that's because he lacks a gift for storytelling, it's because he's stretching it as far as it can go in creating sympathetic yet truly alien (in the general meaning) characters.