Is it a social gaffe to arrive on time for dinner party in Mexico city?

No, it is not considered a social gaffe to arrive on time for a dinner party in Mexico City. In general, punctuality is valued in Mexican culture, so arriving on time shows respect for the hosts and other guests. However, it's worth noting that cultural norms and expectations can vary among different social groups and regions, so it's always a good idea to err on the side of caution and arrive a bit early rather than late.