A pseudo-genius, not clever enough to write -say, an opera- himself, but clever enough to mess about with someone else's.
That's a rather silly comment. As though being, say, a film director was a fallback for those too dumb to write a script.
I'm no opera fan, but I'm pretty sure you don't get to 'mess around with' them in London, New York, Florence, Milan, Berlin, Munich and Zurich if you're 'not clever'. I'm sure he would be the last to bandy round words like genius, but he was certainly an extremely bright and talented man, highly regarded by those he worked with, and for my money anyone people like Peter Cook and Alan Bennett were happy to team up with must have been pretty special. RIP Mr Miller, from one of your innumerable pseudo-intellectual fans.