I suppose it's fair to say Gremlins also had an impact given that it can out around the same time and was also produced by Spielberg. But it was Temple of Doom that really drove the issue, and was the catalyst for the additional rating.

Spaceballs came out later, and while it had some strong language, which was the norm for 80s comedies, and what we can call stylized violence, it didn't contain the sort of realistic violence that was the driver of the PG-13 rating. So, while I would understand if they slapped a PG-13 rating on it, PG also fits with the culture at the time it was released.