All told, if a designer has to pick one or the other, more games can find success with a weak theme and a strong mechanical core. Whereas games with an incredibly cool theme but weak or boring mechanics very rarely stick around for long.
Quotes about theme
While metaphors are fun to play with, players can basically ignore them. […] Games, by the very nature of what they teach, push towards this sort of understanding. Since they are about teaching underlying patterns, they train their players to ignore the fiction that wraps the patterns.