Quote Investigator points out that what really propagates is not a sentence but a template: Writing about music is like ____ about ____, or Talking about music is like ____ about ____. Filled-in variants documented on that page include David Coverdale of Whitesnake (“singing about football,” October 1980), Mike Oldfield (the same, April 1982), John Lennon in his last Playboy interview, and Philip Howard in The Times in August 1991 stacking two at once: “Writing about art is like dancing about architecture or knitting about music.”
There is one honest complication in Mull’s case. In his 1995 book Martin Mull: Paintings, Drawings, and Words, Mull used the line himself — but told it as something he had heard, a story about a painting instructor who told a class that “talking about art is like dancing about architecture.” Quote Investigator notes this may mean he did not originate it, or may simply be a rhetorical distancing device. In 2010 he answered the question the other way: blogger Mike Johnston asked Mull’s art dealer, Carl P. Hammer of the Carl Hammer Gallery in Chicago, to put the question to him, and reported that “Martin confirmed that he is indeed the originator of the famous one-liner.” QI records that this reached print through a chain of three people and says a more direct statement would be desirable. That is why this page says leading candidate and not author.