If he'd been a bad writer without being successful, no one would have cared. But as Aram Saroyan said in "Rod’s Lonely Night," (a piece that Michael Lally, in his own (& as usual generous) post about McKuen, links to), "I should say that poets, generally perceived as ivory tower dreamers and underpaid to the point of extinction, are among the most vainglorious and unforgiving in the matter of readings, appointments, anthologies, and the like, none of it amounting to a hill of beans."
I agree he wasn't treated fairly & poets are jerks, but he it's also true that he was a terrible poet. If I spotted a book of his on a possible's shelves, I went home. I am reminded of a story (that I am probably not quoting accurately), that a Times(?) theater reviewer panned a new play (was it Our Town?). A friend said, But I saw you at the theater & you were crying your eyes out—how can you write what you did? And the reviewer said, It's easy to tug the heartstrings but that doesn't make it art.