With these requirements plz!:

it can be a one-shot

She is considered unpopular/plain/ordinary/average

She can be the love interest or the protagonist in the story

She is not obsessed with finding a boyfriend/romance

She doesn't care too much about appearance

She cannot be in love with 'the popular' guy at the very beginning or too soon, like most cliche shoujo heroines (if that's her love interest, anyway)

No large age gaps, like teacher-student relationships ; they must be around the same age

thank you! 😀

Kare First Love

Rensou Mode (oneshot)

I really liked both, so enjoy!

Nomino
Non-chan to Watashi

Oyayubihime Infinity
Watashi ni XX Shinasai! Personally I don't like this.
Emma

Favorite Art:
Oboreru Knife
19 Days
Gangsta

Sorry, minna-san.. "Non-chan to Watashi" & "Kare First Love" cannot fit the bill. Both are not with glasses most of the time & they're quite "meh~".

I would rather recommend these line-ups:
(oneshots)
Kara no Shounen (Chapter 3)
Shounen Aromatic

(series, some of them are completed)
Hiiro Ouji
Mashikaku Rock
Kouya no Koi
Mishounen Produce
Carat! [Watanabe Yoshitomo] (you might expect no "romance" button, though)
Shinigami Hime no Saikon (very slow romance)
Kuragehime (although there might be a slight age gap in this manga, but still acceptable)
Koi Dano Ai Dano (great one)

Tonari no Kaibutsu-kun

This is the only one I can think of that fits what you're looking for at the moment, but it's quite good.

I second Kuragehime and Mashikaku Rock
Warau Kanoko-Sama and its sequel, Koi Dano Ai Dano
Atashi no Banbi

Quote from Yamiyu

Tonari no Kaibutsu-kun

This is the only one I can think of that fits what you're looking for at the moment, but it's quite good.

thanks but i already read that manga, plus Shizuku doesnt wear glasses 😮

I second, third, x 100 for Warau Kanoko-sama and the sequel Koi Dano Ai Dano really fits your wants and needs
Pika Ichi at first... but she finds out the school isnt what it seems so then... yeah
Ageha 100% doesnt mind her own appearance.. but wants to help others "make up" theirs
Albox - Eien o Kaetara ? may not have enough romance for you
Zombie loan ? may not have enough romance for you
Hyougara Sanka sorta..
Kaitou Magu no Ichibushijuu
Kouya no Koi
Momoko Manual
Shinigami Hime no Saikon she's not considered plain, or aversge.. she's considered the princess of death XD but still... she fits everything else =3
Zen'yasai Patroller

thanks, i read Warau Kanoko-sama and the sequel Koi Dano Ai Dano like halfway, and it think its okay

oh and can i have manga where the protagonist wears glasses most of the time? Not like they USED to wear it but dont anymore, i want them to wear it like 80% or more of the time or smthing like that.

i appreciate your recommendations
thanks!! 😀