Mutsu / Crispin needs another compatible variety nearby to set fruit. Apples almost always crop better with a different apple variety blooming at the same time. It flowers in bloom group 4, which is mid bloom, so a partner has to be flowering at the same time to be any use. Usually grown in USDA zones 5-8.
No single variety stands out as the obvious partner, but that is not a problem here: Arkansas Black, Ashmead's Kernel, Cox's Orange Pippin, Enterprise, and Fuji all bloom close enough to work. There are 6 more in the list below. Pick whichever your nursery actually carries. For home plantings, aim to keep partner trees within about 50 feet if possible. Nearby crabapples that bloom at the same time can also help.
One thing to keep in mind. Triploid. It is usually a poor pollinizer for other apples. Triploid means this variety has pollen that usually does not work well for pollinating other trees.
To put that into practice: at a minimum, plant two different apple varieties with overlapping bloom, or one apple plus a nearby ornamental crabapple that blooms at the same time. Better still, plant two to three different apples with overlapping bloom windows so each tree has more than one workable pollen source. If you already have a poor pollinizer such as a triploid apple, add a dependable diploid apple nearby instead of counting on the existing tree to do both jobs.