cosmetic bugs are nothing but having low severity from testers point of view and loe priority from business point of view. These are the bugs which doesn't affect the functionality directly, cause no harm to system, dont result in any failure.
Issues which are of Low Severity which don't effect the functionality of the application.
Priority of the Cosmetic issues depends on the situation. If the issue is found in Pilot(production) then it is of higher priority than found in Test Environment.
Ex : UI Issues which don't effect the functionality are a good example.
Consider the following example, per requirement Logo in the banner of the page shall be of red color. It has been missed during system testing and has been identified in pilot as pink(log0). The priority which would have been assigned in Test Environment was clearly p4, as it is found in production it becomes p1.