Hi nksyoon,
Yellow Fin from the Indian Ocean is perfectly fine for Sushi as long as it has been caught, frozen immediately, stored and transported under strict guidelines and required temperatures. I have a friend who is in this business.

It's not valued as highly as Blue Fin but eats very well in my point of view. A 'freshly frozen' Yellow Fin is far better than a mishandled Blue Fin. The key is to get fish in the best condition possible. That is Sushi!