Speaking purely as a rose gardener, not as a philosopher, there are plenty of beautiful roses without thorns, or with teeny-tiny thorns like little stickers, and very thorny roses that aren't much to look at. Thorniness doesn't have anything to do with beauty in roses.
Not to mention all the beautiful flowers without thorns, like violets, lilies, peonies . . .
My guess is that this is a metaphor for beautiful women who laugh at philosophers, BWDIK?