Thanks for the link, Loon, but it's clear that the words "cloth ears" are literally true in the case of Pepys's diary entry, given that it reads: "found with cloth ears on and a false mayne, having none of his own." That is, the horse was 'done up' to look normal despite its disfigurements.
Clearly, I should in my earlier response have said, "The earliest recorded use of the exact phrase 'cloth ears' - in the sense of being hard of hearing or inattentive - is in a book..."
I apologise for my lack of precision, but I presume your mother refers to the use of the phrase as I have just defined it rather than as Pepys means it. Cheers