Sushi is more of a way to prepare a food than it is the food itself. As such, it depends on what the food is when you ask is it low fat. Typically Sushi ingredients are high in protein and carbohydrates. So yes, it is basically, by definition of the process, low in fat, but that can change based on the ingredients used. A slab of fatty tuna would increase the fat content, while a rice roll will always be low in fat.
It’s kind of like asking is boiling low in fat…….well, that depends on what you’re boiling and how you serve it.