Is Mark Henry a jobber? He's been doing the same thing as Miz lately. Well, no, neither man is a jobber, especially Miz. They've acknowledged his "losing streak" on TV, so I'm assuming they have a direction for him; they want to weave it into a storyline. I mean, yeah, he's losing, but it's to some serious talent. CM Punk, John Cena, Big Show, etc, they're not scrubs. He's losing to main event guys. Mick Foley made a career out of that, but you'd never call him a jobber or anything of the sort. He's putting other guys over right now, but would you be surprised if he worked his way into the WWE Championship scene after WrestleMania? No, he's fine.
Miz isn't a jobber, he's just not winning. Putting people over is fine, but I don't think a former WWE Champion who is consistently losing to high level opponents can be put on the level of a Jinder Mahal, a Yoshi Tatsu, or someone like that. Daniel Bryan, the World Heavyweight Champion, had been losing matches fairly consistently until a few months ago, the same can be said for many guys on the roster. Miz is a high level talent who they can use in many capacities, he's not a jobber, though.