The other characters are quite perplexed about the idea of 3D, too, not just the stupid Homer. A scientist gives a brief lecture, similar to how scientists IRL introduce 4D.
Anyway, that answers the good old question "Is the gruesome cartoon Itchy & Scratchy photorealistic from the Simpson's POV?"
Photorealistic: yes. However, it could be debatable whether it's gruesome. We see situations that characters survive with short term damage but no long term consequences (example: Homer skating into the canyon). So while it would be gruesome to us, it's probably closer to slapstick to them.
And the first thing he did in the 3D world was go to an erotic bakery. I always thought it was a dumb throwaway joke but after thinking about it, that would be a fun first experience in the 3D world