I try to make a study of the metaphors I come across, and assign them a letter grade. Here's one, from Gail Collins in the New York Times today:
"This will certainly require all the moral suasion available to the minority leader, Mitch McConnell, a man with the natural charisma of an oyster."
I rate this a C-. It's true that oysters are less charismatic than, say, Clark Gable. But "charisma," in the sense Collins is using, means "a special quality of leadership that captures the popular imagination and inspires allegiance and devotion."
Now, I don't know about leadership. Oysters cannot command mobs. But if you don't think oysters can inspire allegiance and devotion, get yourself a subscription to Saveur. Among all things, why oysters? I'd argue that almost any given tidepool creature, a limpet for instance, is less "charismatic" than an oyster.
But the image did stick in my craw, so it passes.