If you are asking if the length of the password by itself be sufficient to create a secure password, then the answer is mostly no. You need many iterations of the hashing process otherwise brute force attacks become trivial given today's hardware.
What we really care about is how hard it is to determine the passphrase given the hash. With a 128 bit hash, an attacker requires an average of 2^127 guesses if they are guessing completely randomly. So as long as your passphrase is well before the first 2^128 guesses an attacker is likely to make, making it harder to guess is theoretically useful.
For example, "AAAAAAAAAAAAAAAAAAAAAAAAAAA" has more than 128 bits, but it's also going to be (relatively) easy to guess. As shorthand we say "it has less than 128 bits of entropy"
In the example that GP gave, you could advertise "I used 10 diceware words for my passphrase" and it would still be as hard for the attacker as attacking a 128 bit hash. 7 diceware words would be much longer than 128 bits, but if you advertised "I used 7 diceware words" it would give the attacker a significant advantage, since there are much less than 2^128 possibilities.