"Mutt" is just a word, and
by definition any non-purebred dog is a 'mutt.' So it's an accurate phrase. Not everyone who uses the term is doing so to be condescending or implying that any one dog is lesser than the next and not everyone who reads it takes offense to the term. I honestly will never understand the oversensitivity some people have to the word.
Heck, I affectionately call my dogs "mutt" all the time.
I have NEVER seen a case here where any one dog was viewed as "lesser" becuase it was not a purebred. I have seen many instances where people express their disapproval of purposely mixing the breeds though...something completely different. I think that sometimes people go on the defensive and confuse the two though.