From my meagre understanding of climate, I think the mountains would shield the area to the east from winds bringing moderating temperatures from the sea to the west (you can tell the winds are coming from the west because they're dumping all their rain on the western side of the mountains and making all those rivers). So beyond the mountains you'd have warmer summers and colder winters than on the seaward side of the mountains - i.e. a continental climate rather than a temperate oceanic climate. So yes, if you want it to be colder in general a big plateau is probably the way to go.