It seemed an odd concept on the face of it: Pop diva Mariah Carey on MTV Unplugged? Since when is a voice ever plugged in? But why quibble: Carey's March appearance was a vocal tour de force, proving beyond a doubt that in addition to her breathtaking pipes, she has developed a commanding stage presence. Besides nailing such hits as ''Vision of Love'' and ''Emotions'' with ease, she turned in a killer duet with Trey Lorenz on the Jackson 5's ''I'll Be There,'' which became a No. 1 single and pushed the EP of the event into Billboard's top 5. In the process, this rare public appearance (now on video) reminded us there was a big-league performer inside that party dress.