You can (and you could even use tarsnap to back up the encrypted filesystem image if you want), but writing your data to an encrypted filesystem tends to expand the amount of data changing -- in the extreme case, if you create a copy of a file you'll write that many blocks of new encrypted data which needs to be backed up, whereas tarsnap would just say "hey, I recognize all these blocks, it's those ones I backed up earlier" -- so Tarsnap's encrypted backups of a filesystem tend to be many times more efficient than backups of an encrypted filesystem.