Given the disparity in available forces, mainland greece was doomed. The british could have held crete, with a few different decisions, but even that fell otl. And note that crete is in the middle of the sea where the brits had a navy and the germans didnt. It was also far enough off shore that the luftwaffe was at a distinct disadvantage. And the still lost it.
Could they have done a better job holding the peloponnese? Sure, but it would have just held off the inevitable for a matter of weeks, imo.
Anything north of the corinth canal, eg athens, is asb. Imo.