I don't know how you can reduce the garlic flavor of the pimiento cheese once you have mixed it in other than to make another batch without the garlic and combine the two.
However, as my husband and I are kind of nuts about the stuff ourselves, we both agree that true southern pimiento cheese should have no garlic at all.
Here's the recipe for Pimiento Cheese that I always make for parties. Most folks here in NY do not know what they are eating as it's a southern regional thing, but they rave about it all the same.
1 1/2 pounds shredded cheddar cheese (I mix half white extra sharp and yellow NY sharp or use other bits of cheese I have laying around-jack, pepper jack, provolone, etc.)
1 4 ounce jar pimientos, drained and coarsely chopped
2/3 cup mayonnaise
2 tablespoons apple cider vinegar
1 tablespoon honey
1/2 teaspoon ground cumin
1/2 teaspoon coarse ground black pepper
1/4 teaspoon kosher salt
2-3 shakes Tabasco sauce (or more if you like)
1/8 teaspoon cayenne pepper
1 pinch nutmeg
In a large bowl, combine the grated cheese and pimientos. In another bowl whisk together the remaining ingredients; pour over cheese mixture.
Stir together with a rubber spatula until the cheese is nicely coated with the sauce. It should not be stiff, like a cheese ball, but loose, like a spread. Chill for about an hour to allow flavors to blend. Serve on crackers or on celery sticks. Pim-Chee sammiches are great too with a big mound of crunchy iceberg lettuce. | 
