I have what I would call "mirror vibration" at highway speeds. It's really not enough of an issue for me to worry about. I guess if there was a specific fix that reliably made the mirror dead steady at all speeds I would let them do that, but my impression is that even if I complain and get it serviced the best I could hope for is "slightly better." I'm assuming the issue is in the mirror mount itself, and unless/until they do a redesign of the entire part having it addressed is pointless.
My suspicion is that there was an earlier problem with "mirror shake", where the entire housing was shuddering at highway speeds, and to address this Mazda added bracing in the door to reduce flex at the point of attachment. Being aware of this, owners started staring at their mirrors and noticed a little vibration and assumed that this was the dreaded "mirror shake", but I can't imagine that the very slight issue I see would have necessitated changes to the door design.