Another break-up song, ‘Mr. Perfectly Fine’ is widely believed to be about Joe Jonas, who Taylor dated briefly in 2008. In the track, Taylor speaks directly to a recent ex who dumped her without explanation. She seems to resent him for being “perfectly ¬fine” after their split and moving on quickly with a new girl while she’s still heartbroken. Taylor also calls the ex “casually cruel”, a description also used in ‘All Too Well’ from Red. Jonas’ then wife, Sophie Turner, showed her support for the track on Instagram Stories, writing: “It’s not NOT a bop.”
I would like to know what does "It’s not NOT a bop" here mean. Thanks.