Do you think it's wrong to mention that the under arm hair on my daughter-in-law is gross and disgusting?
What do you find gross and disgusting about her hair?
I'm often telling her that her hair is a mess.
It's her personal decision whether she removes her body hair or not. This includes areas like her underarms or pubic hair.
You are saying that it's okay for women to have body hair.
How your daughter wishes to physically appear is up to her, though there is no harm is giving her suggestions and hints as to how you might want her to look. Take a passive but sincere approach and your daughter in law will appreciate the concern.