Even ignoring the fact that Klingon K-Pop was just a fun and funny scene which should be all the justification needed for choosing that it also fits with the episode's premise better than Klingon opera would have. We know Klingons sing while on duty, whether it be opera or battle shanties or whatever. That's nothing unusual and thus would not make them want to show up and attack an anomaly to free themselves from singing opera just so they can go back to singing opera. Singing and dancing like a K-Pop band? That is unusual and probably threatens their honor (because reasons). That is a reason to go attack some cowardly and dishonorable anomaly for the glory of the Empire or whatever.
Remember, the point of this episode is that the singing is unusual and out of place. Klingon opera in a Klingon ship would not have been either. K-Pop on a Klingon ship on the other hand...
Remember, the point of this episode is that the singing is unusual and out of place. Klingon opera in a Klingon ship would not have been either. K-Pop on a Klingon ship on the other hand...