There used to be something like that when I was in early high school. Called stripcreator or something.
IIRC, It let you make 3-panel webcomics using characters from well known, successful webcomics. It was very web1.0, but it worked and my friends and I spent a lot of time making stupid comics.
Don't think it exists in the same form anymore because the people who owned the characters probably got mad...and the users would have wanted fancy AJAX menus. It did introduce me to some webcomics that I read for a long time though...Diesel Sweeties was one (although I have long since given up on it)