There are some names that are considered nn's that bother me used as full names and some that don't. I'm prejudiced like that, I guess. :b This one bothers me. But that's partly because I love
Theodore so much more than
Theo, and I prefer
Ted,
Teddy, &
Ned as nn's for it over
Theo anyway. :) But I feel like
Leo can stand on its own, so I don't see why
Theo can't. It's less "nickname-y" than
Millie,
Maggie, or
Evie, imo, which are used more commonly in the U.K. as full names than in the U.S., too.