His response

“ Trung: there are others that "aspired" to be serious composers: Keith Emerson, Rick Wakeman, etc., but not one of them got past a rather pretentious brand of rock 'n' roll and could be considered a genuine con cert composer except Frank Zappa. He was, and remains, unique. ”

There you go at least one composer/musicologist rate Zappa as a composer.

I may well have to give Zappa composition a listen although I suspect I will dislike it due to my anti-modernist attitude