Sugar-free jello contains small amounts of maltodextrin, which is a glucose polymer. This can be both good and bad. On one hand it's a sugar so it may kick you out of ketosis. On the other hand, if you're a ketogenic dieter, it's one of the best kinds of carbs to eat because it breaks down to glucose and muscles absorb it very well (unlike fructose which gets stuck in the liver for conversion to glucose).

Ultimately it probably depends on quantity. A single cup of sf jello may not hurt, but... can you eat just one? I can't, so I just pass on it altogether.