NEW YORK - The disarming sincerity of Joni Mitchell capped a fine program at Fillmore East in the second show on April 25. This second of four performances of the program had an impressive opening with Taj Mahal and a strong, but overlong, set by the James Cotton Blues Band.

Taj Mahal began with an a capella number, then accompanied himself on acoustic guitar in "Good Morning, Little Schoolgirl." Three backup musicians joined him for the rest of his set, which drew heavily on some of the best material in his latest Columbia album, including "I Ain't Gonna Let Nobody Steal My Jellyroll," "The Cuckoo," and "She Caught the Katy and Left Me a Mule to Ride." "Corina" was a big building number. Taj Mahal's homey blues voice consistently hit the mark.

Cotton, one of the best harmonica players around, played number after number ably assisted by his solid group. "Duke Patrol, "Stormy Monday Blues" and "Knock on Wood" were but three of the good selections. Luther Tucker, as usual, contributed top-notch guitar work, while Barry Smith was strong on drums. Saxophonist Bill Nugent and bass guitarist Bob Anderson also were solid.

Miss Mitchell began her set with one of her top compositions: "Chelsea Morning." The high quality never let up. The gentle "Cactus Tree" was followed by the lilting "Night in the City." The telling "Nathan La Franeer," the powerful a capella "The Fiddle and the Drum," the beautiful "Morning Morgantown" were all included.

"Let's Get Together" was the only number the Reprise artist sang that was not composed by her and it was turned into a sing-along, probably a Fillmore East first. Her closing selection was her biggest song, "Both Sides Now." It drew a deserved encore, which was "Michael From Mountains."