Well, her singing was meant to be used as a demoralizing weapon. In that, she succeeded.
To be fair, Minmei's Japanese voice actress was actually pretty decent (I've heard better, but it wasn't bad). Her American voice actress had apparently never been asked to sing before in her life. Though to be honest, I never liked Minmei as a character period, no matter what language she spoke in. She was supposed to be this underage hotty idol singer, and she was just whiny as hell. No wonder the Zentraedi wanted to destroy the SDF-1.
Of course, if you're a fan of the McKinney books like me, the entire Robotech Saga really was Minmei's fault.