Customer complaints are a sign that a product needs improvement. Sometimes that's because a product doesn't work, but other times it's because users are pushing the boundaries of what it was designed for. When basic functionality is intact, complaints can be seen as a measure of engagement. If users weren't using the product much, it'd be easy to just switch to another tool instead of filing complaints.
In fact, it's a bad sign if users aren't complaining. It means that they can't imagine any feature improvements for your product and that they can't imagine receiving any additional value from your product, implying that they don't see any room for growth. While it's never fun to hear people complain, remember that silence from customers should be even more unnerving. To keep this in mind, measure complaints like any other engagement metric.