This metaphor is powered by a particularly vivid contrast: the inviting sensory appeal of a festive beverage juxtaposed with the revolting suggestion of feculent contagion. Therefore, labeling someone a turd in the punch bowl is most appropriate when the individual's deleterious influence goes beyond mere faux pas or nuisance behaviors, and rises to the level of deliberate offense for its own sake. Consider that the literal act of depositing or excreting fecal matter into a communal food-service container would be sabotage.
The punch bowl and the feces connote certain additional nuances. The former is a symbol of public community, as such dispensers are frequently encountered at parties where they become a focal point for interaction. Freud famously identified feces with aggression and the possessive instinct. Thus a turd in the punch bowl suggests rage toward, and / or the urge to conquer, a community or society as a whole. Defecating into a punch bowl is a very public act, in contrast with poisoning the well or laying an upper decker, which are generally surreptitious. In particular then, to be a turd in the punch bowl
2. Similarly, a gauche, socially inept or unwelcome person who has a stultifying effect on social gatherings, or, by extension, was involved in some futile or hugely unpopular effort.
1. -- "All I did was tell one harmless little dirty joke and the whole room went silent. Apparently I had crossed some kind of taboo. No one even chuckled at the joke; it was a veritable turd in the punchbowl. I felt like one too, since the whole party immediately seized up and everyone stared at me."
2.(a) -- "There we were discussing local politics, and who shows up uninvited and parks himself right in the middle of our group? Little Georgie, the mayor's brat, who has an unerring sense of inappropriateness. I'd rather see a turd in the punch bowl than have to deal with that nuisance."
(b) "Practically everyone in town was in favor of a weeknight youth curfew, but as usual our "freethinking" pastor went contrary and opposed it. He didn't change anyone's mind. I think he's a jinx! A turd in the punch bowl of civic affairs."