South Boston is ready for restaurant diversity. Kendall Square, in the same situation for years, suddenly has a new place on every block. Both neighborhoods have restaurants specializing in sushi: Moko in South Boston and Fuji at Kendall in Cambridge. Both are already popular, filling a gap in their neighborhoods. But they remind us that pretty decor does not always equal excellent food. With seafood sustainability a concern, perhaps it’s time to stop treating sushi as everyday fare.