Why it says so much about quality
Because the customer decides it, not you. Resolution time can be dressed up by closing quickly; a customer coming back cannot. Many reopened tickets with very good resolution times mean one thing: cases are being closed before they are resolved.
What to look at
There is no figure that works for every industry. What is useful is seeing which ticket types get reopened: they tend to cluster in two or three, and that points to a very specific process or training problem.
The usual mistake
Treating it as an individual agent’s failure. Reopened tickets are usually a process issue: missing documentation, pressure to close or a type of fault that nobody knows how to fix well.