Mutt is just a description of a dog . All dogs come from mixing something together , so no big deal. I have a "mutt" and a "pure" Yorkie , I love them both and don't value one over the other.
 
Just try and not to read to much into a post. More often than not intent doesn't come across in a post. Such as the example someone gave..."How big will my Morkie get?" If someone post and says, "no one knows because its a mutt." They may be sitting behind their computer with 
absolutely no ill will toward your dog or the word mutt , but when you read it you see " It's a mutt , lesser than my dog , and your not worthy to be here."

 So try not to take most things in a bad way.  
The post that are obviously mean , that's another story