I was wondering if someone can me how to cut the sweetness down in my buttercream?

What is sweet to one is not to others.
Have you tried adding some salt? About 1/4 to 1/2 teaspoon per batch of icing (using 2#s of sugar) helps.
Some change the flavoring used in the b'cream.....use something with a lemon taste helps. Perhaps one or both of those suggestion will help you.

I use a recipe with heavy whipping cream (not water), a 1/2 tsp. of salt per batch, and I also add about a tsp. of lemon JUICE per batch. It cuts the sweetness a lot.

Rae