the term "mutt" is used to indicate that the betta is of unknown parentage. that's not to say that there's anything wrong with keeping mutt bettas as pets or that mutts can't be absolutely beautiful little fish
the thing is, there are countless mutt bettas dying on the shelves every single day. not because there's anything wrong with them, but because the supply has vastly outpaced the demand for many years now.
what that means is that there's not a lot of reasons to breed mutt bettas and plenty of reasons against it. this is true of all mutts, whether they're crowntails, veiltails or otherwise.
sadly, many people don't really understand the difference between a LFS betta and one from a reputable breeder; it has nothing to do with tail-types and everything to do with genetics. just because a betta has a crowntail doesn't make it special; there's also nothing wrong with a quality VT (there is still a competitive market for those, btw).
long story short:
only breed bettas that have a family tree. if you don't know it's parents and grandparents, then you've got a mutt on your hands and this world's got too many dying mutts already.
--EDIT--
actually, let me add that i'm also don't support the casual breeding of any bettas except under specific circumstances. i really think that it should be ( a ) an exceptionally beautiful pairing (not just the typical betta beauty

), ( B ) a pairing intended to further the development of a show line or ( c ) when one or both of the parents are award-winning. i recognize this is not a popular attitude and would offend many beginning betta breeders, so i typically keep quiet about it.
there are just so many bettas dying on the shelves that would be beautiful pets.

i know that the vast majority of you who breed just for fun "already have homes arranged" for the babies. but i can't help but think that most of those homes wouldn't know or care about the difference between a show-quality betta and a healthy mutt. every home-bred baby that goes to an owner like that is taking the place of a mutt on a shelf somewhere.
i know that no one wants to hear that guilt-trip when they're busy being proud of their breeding attempt, so i don't say it. but if i think too hard, it makes me sad.