Yes, you can find sushi and sushi style items on Disney Cruise Line, but it is fairly limited and not something you will see every day or on every ship.
Some of the main dining rooms offer dishes that are close to sushi or sashimi rather than traditional rolls. Enchanted Garden, for example, often features a very good tuna tartare. Royal Palace has an ahi tuna salad, and Animator’s Palate commonly serves an ahi tuna steak. These are popular options for guests looking for lighter or seafood focused dishes, even though they are not classic sushi rolls.
You can also sometimes find sushi at Cabanas during lunch. On most ships, it appears on one day of the cruise and is usually limited to a small selection. Guests often report seeing a few pieces of a California roll or another simple roll rather than a full sushi spread. It is nice to have as an option, but it is not usually a highlight of the buffet.
If sushi is very important to you, it is best to think of Disney cruise sushi as an occasional bonus rather than a regular offering. The quality is fine, but it is limited in variety and availability. For most guests, the tuna dishes in the main dining rooms end up being the better and more consistent choice.