Okay, so why do they dress this way?

(not that I am complaining)

girls might WORK in theme restaurants , if her face is older looking, but in Harajuku, Tokyo, dressing up and visiting Harajuku, Tokyo from hours away is a big deal on weekends for teenage girls.

Harajuku, Tokyo, especially near Jingu Bridge, especially on a Sunday

A Beginner’s Guide to Harajuku’s Lolita Fashion:

https://savvytokyo.com/a-beginners-guide-to-harajukus-lolita-fashion/

Cosplayers are people who dress up as anime, manga and cartoon characters, in specific dresses or as members of popular bands. Lolita fashion is also popular in Harajuku, ranging from a wide range of genres including gothic lolita, fairy lolita, punk lolita, and the classic lolita

from https://jpninfo.com/5912

Mostly to get attention. There are maid restaurants, but the girls working there will only wear the dress inside. The girls wearing stuff like that outside are most likely doing cosplay. Which means to dress up as someone and maybe have strangers take pictures of you.

Looks like pedo bait to me.

Is there a subculture in Japan of sexual deviants who like young looking women dressed up like a french maid?

It's somewhat too common to be only considered a subculture, though, it's mostly restricted to Tokyo. And it's not "young looking women", but actual schoolgirls who do cosplay and have jobs at maid cafes.

What are maid cafes?

Coffee / tea shops where the staff dresses like this?

Yes, but they not only dress like maids, there's also some acting involved. Like treating customers as the master of the house.