Joni Mitchell told her Convention Center audience last week that fame has its price and "you constantly have to keep things in balance."

One way she handles the problem, evidently, is by isolating herself. Members of the press were told emphatically by her manager that "Miss Mitchell does not grant interviews." A Times photographer was hustled from the orchestra pit by a stagehand who worried that the photographer's presence would unnerve her.

His concern may have been justified. Mitchell's appeal does seem to derive, in part at least, from a nervous vulnerability. It wasn't until the second-half of the show that she worked up the confidence to talk, in rambling and uncertain fashion, about her songs and her uneasiness in unfamiliar surroundings. The large, adoring crowd reacted with predictable warmth and sympathy.

But her apprehensions seem unwarranted. Mitchell is immensely talented. She is a gifted singer, songwriter and artist; her concert was a tremendous success. Her voice was strong and clear and her music, especially the recent stuff, was excellent, the fruit of her continuing relationship with Tom Scott. (Incidentally, Scott's L.A. Express was excellent, another success story in the making.) Visually, Mitchell was devastating, a sylph in silver silk. She has a delicate, radiant beauty as appealing to women as to men.

Anyway, all Mitchell needs now to keep things in balance is a few problems.