So given that most romance manga are wish fulfilment vehicles, where the love interest is basically perfect, I am looking for a Shoujo or Josei Romance manga (i.e. a manga that has the female audience demographic as its target) where the ML is ordinary.

I really don't want a manga where the ML is pretty much perfect (good looking, good grades, rich, lady killer etc). I want to read a manga where the ML is just an ordinary person without any mob characters going Kya~ Kya~ over them.

Hobbies include impersonating fictional characters and sucking on ketchup packets.

Futago
Shall We Have Dinner Tonight?
Toshokan Sensou - Love & War
Get Love!! - Field no Ouji-sama
Hana to Rakurai
Socks Goblin

3D Kanojo

Hibi Chouchou

Hirunaka no Ryuusei

Issho ni Neyou yo

Kimi ni Koishitei desu ka

Ran to Haiiro no Sekai

Monkey High

Here it shows only 1 chapter translated, but it can easily be found translated in full (31 chapters)

I would definitely recommend My Hero!
and maybe Cinderella Shindoro-mu (Onshot subversion of the trope) as well.

I believe I can link quite a few, but some (most?) have age gap. I hope they match what you're looking for, I've read these and would recommend them. Also, I'm sorry for the long post ^-^

Can't See Can't Hear But Love - it's a seinen, but it's good (the rest are shoujo or josei). Get tissues ready though.

Houkago x Ponytail - not sure if this fits cause he is successful in what he does, but... he doesn't have groupies. It's very cute.

Ojikoi - normal dude, high school girl

Perfect World (ARUGA Rie) - well, it's not a mary sue ML for sure

Sekine-kun no Koi - he's awkward and she's average, they're cute together

Go Isshou ni Heart mo Atatamemasuka - he's a convenience store manager

Kawaii Hito (SAITOU Ken) - he's a florist, a "scary" one, it's cute AF

Wotaku ni Koi wa Muzukashii - the cover they chose isn't great, but It's mainly a casual romance/comedy/slice of life about 2 otaku couples (and soon to be 3 with the added characters)

Itsuka no seishun - I couldn't find it here
Amber - again, couldn't find it here, he's a writer who doesn't take care of himself
Darenimo Ienai Maruhi - same, he's a widower

Kimi ni Koishite Ii Desu ka. - not sure this fits the search

Mizutama Honey Boy - 2 cute high schoolers

These ones I like, but they might not be what you're looking for:

Kore wa Koi no Hanashi - older man, young girl, timeskip

Watashi no Shounen (TAKANO Hitomi) - like the one before but the boy is young and she is older *seinen though, this one (couldn't resist linking those two)

Koi wa Amaagari no You ni - personal favourite at the moment, it even has an ongoing anime

I live for good age gap romance.
...
Wubba lubba dub dub!