Its 2025 and still no update to mass update customers using an import file, just how it is done to products. I don't see what's the big issue in implementing the exact same feature for customers. 

Since you assign ID or track products with their names - similarly you can attach an ID or track customers by their name - this creates a Unique identifier for each customer (ID or their name).

After this, we can easily export our customers, make changes and then import that file back to QBO. Once it loads, the changes can be updated to existing customers by matching the unique identifier (ID or their name).