usually i'm all for out there nature names, but Thistle i think is best for a character or a pet (would be lovely for a cat).
for a person, it feels too unsubstantial. most other flower names have clearer meanings and imagery. thistles however are barely even known.
also, though i love names with 'th' in them now, i was unable to say that sound as a kid (had a few speech impediments + a leftover, watered down accent from immigrant parents).