It's not that it's evil, it's more so the intent. Vegeta is basically saying he had the option of bringing in something fantastic to Krillin's BBQ. Like some expensive or bountiful meat. Instead going with the cheapest and easiest option because he either couldn't be bothered to, or because he still likes to mess with him.
The joke is that literally any other potato based food would be better. Even a dollar store off brand bag of potato chips would be better. He picked it precisely because it's the worst thing you can bring.
I understand if the potato salad is nasty, or blend, or they put raisins in the potato salad but other than that I don’t see anything wrong with somebody bringing Potato salad
Could be a throwback to a line in hellsing ultimate abridged where a vampire who killed the main characters friends and turned them into zombies says "all your coworkers are bringing is their own rotting flesh! Still better than potato salad if you ask me,"
