The phrase "disappointed customers" means that people who bought or used a product or service are not happy with it or their experience. Full definition
Retailers can also avoid disappointed customers by asking questions in the store to point shoppers towards the best product for their pet. (petbusiness.com)
But no service provider can afford to disappoint customers for very long. (sciencedaily.com)
Otherwise, you'll get some REALLY nasty reviews from disappointed customers. (writingnovelsthatsell.com)