The only thing I could find on Plant dissing other artists was this piece from Digital Spy, 21 Jan. 2008:
Led Zeppelin's Robert Plant has complained about Radiohead and Red Hot Chili Peppers after hearing their records in a North London bar.
Music by the two rocks acts was played at Camden's Fifty Five Bar, prompting Plant to complain and request they play '60s psychedelia star Captain Beefheart instead, according to The Sun.
"He was drinking with a woman and didn’t like the choice of tunes playing," said a source. "Radiohead was on and he started complaining. He said, ‘What’s this rhyming cr*p?'
"The staff were obviously keen to please him so they changed the music. They put on the Red Hot Chili Peppers, who they thought might be more up his street.
"But he didn’t like their stuff either and said it was like a ‘nursery rhyme'. He then said he wanted to listen to Captain Beefheart."