Wednesday (February 23) was a happy evening at Carnegie Hall: Joni Mitchell was back. Miss Mitchell, who did not play Gotham last season, still was cheerful and relaxed. And, again, she was the best interpreter of her own songs. That's quite an accomplishment considering how many times tunes such as "Both Sides Now (Clouds)," "Woodstock" and "The Circle Game" have been recorded. The Ron Delsener promotion grossed $16,000 at a $6.50 top, a sellout.

The minute Miss Mitchell appeared, much of the audience were on their feet cheering. But they were soon quieted as the artist went into her first song. She balanced her program between old faves and newer selections since the concert was being recorded for her new label, Asylum, where she switched from Reprise.

The Canadian gal, whose programs used to consist mainly of numbers backed by her guitar playing, has improved at grand piano and added a dulcimer. She spends about as much time tuning the latter as she does her guitar but her friendly visage helped these moments pass.

"The Circle Game," her second encore, was a singalong, a tradition for this number. This time, Miss Mitchell also had associates and friends assist her onstage, including Jackson Browne, the opening artist. Browne, who also records for Asylum, gave a short set, which included some of his familiar tunes. He was a good opener for one of today's top femme folk -style artists.