Mid-Tier Stores の意味は？

「高級品とまではいかないものの、そこそこの品質として定評のある(中級)品を色々取り揃えて販売しているデパートなどの小売店」ということになりますね。
日本語では特に呼び名はないかと思います。

Mid-Tier Department Stores means stores that offer products for sale in a broad assortment of unrelated product categories, which products are generally classified in the trade as "better" (but not "best") quality products. Examples of Mid-Tier Department Stores include, without limitation, JC PENNEY, SEARS, MERVYN'S, STEINMART, KOHLS, FRED MEYER, THE BAY, CLEMONT and SIMON'S.
https://www.lawinsider.com/dictionary/mid-tier-department-stores