Here's a weird one...
Someone mentioned it a few months back and I looked it up now. Apparently back in Argentina back in 1978 they published an unlicensed and very odd bootleg Wonder Woman comic based on the Lynda Carter TV show, written and illustrated by brothers Jorge and Mario Morhain in collaboration with Gene Quinterno. Issue number one seems to be lost.

And that's weird enough, but this is funny: each issue contained a silly parody of Wonder Woman called Little Wonder. Seems to be a sort of midget or child Wonder Woman who is much sillier, which really makes me think of Teen Titans GO!

Little Wonder! 28 years before Wonder Loli.

It's obscure and I'd say this is the first piece of fanart ever based on the character, but I can't say that because the comics themselves encouraged kids to draw their own versions of the characters and mail them. So, well, here's the very first internet era piece of fanart.