Any combination of dogs is considered a mutt. Mixed breeds are mutts. Purebreds are not. Mutt seems to have a bad conotation to it, like they are "lesser" dogs, but they're not. They're just mixed breeds, hybrids, mutts, heinz 57 or whatever..lol.
Looks like a boxer mix puppy to me, but it's hard to tell with puppies..as size is a factor and the picture doesn't show the whole dog.