I kinda like the fact that Ataru is looking quite average. Not ugly, not cute. Just normal.
She's not goth but I think her looks are in a similar vein as to why I find goths hot. Also freckles are something I always love seeing on characters. She's not sexualized or cutesy, but she's still got hot points to people like myself.
Ataru's look is classic 90s dyke (to watch out for) to my eyes – the short, messy hair, the understated, serious glass frames, the casual jeans + shirt combos. I don't think she looks plain at all. Especially next to moe blobbish characters like Wanko, she really stands out. Somebody with her sense of style would have been a classic ladykiller in San Francisco in the 90s / 00s.
Her relationship with Kyouko is also very SF 90s and 00s, but in a different way. Good luck, Ataru! You're gonna need it!