Frozen fish does not make for good sushi or sashimi...............
Perhaps it is better fresh, but almost all sushi served in the US is previously frozen. If a US sushi chef told me it was not ever frozen I’d figure he was lying and never trust him.
If you go to the main seafood market in Tokyo you will see tuna, among other fish, coming off the boat already frozen. They have an interesting auction of frozen tuna carcasses early in the morning.