When someone or something is a full on monet they are pretty from far away but ugly from up close
EX: someone seems nice or pretty but is not a very nice person(vice versa)
the saying comes from the ICONIC movie Clueless
Cher—who is pretty much like "as if"—tells her, "She's a full-on Monet," and Tai is all, like, definition please, so Cher elaborates, saying "It's like a painting, see. From far away it's okay, but up close it's a big ol' mess."
Full on monet means that someone is pretty from far away but ugly from up close.