A lot of factors. The direction the mountain faces, proximity to large bodies of water, etc. For instance, there is the infamous "Stratton Cloud." The top of Stratton always seems to be cloudy, but if you look across the valley, Bromley is basking in sunshine. Also, the mountains are covered in deep snow which radiates the cold. If the air is warmer, it's going to generate fog/clouds.
I actually experienced more bluebird days last season in Vermont than I can ever recall. I don't see the Vermont weather as being generally cloudier, just more likely to be cloudy around the mountains. But I could be imagining things.