I agree with your dating of the 1920s. It is very pretty! Many hat linings/labels are not necessarily the name of the manufacturer or hat company. Often the name on the lining is a "line" of hats that a hat manufacturer would make for a certain year or season. So "Blue Mound" might be the group of hats made that year or season, to appeal to a certain type of woman or to her budget, etc. Although it could be the name of the hat company, I am not familiar with that name. Maybe others might know more if they have access to online subscription research sources.
I have not seen Blue Mound on a hat in my many years of collecting. Great to see.