Carly Simon wrote a whole lot of excellent songs, in my opinion, and she has performed them very well too.

No, I don't rate her collected body of work as highly as I do Joni Mitchell's...but I see no reason for her to feel unconfident about her own musical work which has always been very good, certainly better than most.

You can make a ketchup commercial out of any song. From any writer. One of Dylan's songs was used for a bank commercial. One of Donovan's was used to sell some kind of spread for putting on toast. Commerciality being what it is, it's probably inevitable that a great Joni Mitchell song will someday be used to sell spam or hair spray or something... If so, it will not be any reflection on Joni Mitchell. Neither is a ketchup commercial any reflection on Carly Simon.

Carly has another claim to fame too. Her lips. ;-)