Yes, it is often foggy in the Blue Mountains, particularly (for some reason, presumably due to the local topography) at Katoomba, which seems to get more fogs than the towns higher up or lower down. I have no statistics though on the number of foggy days.

Summer is no guarantee of clear weather: I have spent two weeks in the Blue Mountains in January and each day has dawned drizzly or foggy, and cool.

It can be a fine day in Sydney but foggy in the Blue Mountains. However, if there's a westerly wind blowing in Sydney it will almost certainly be clear in the Blue Mountains. In the winter it will also feel freezing up there, but the visibility will be excellent.

I ought to add, in fairness, that I have been to the Mountains on many occasions and had no trouble with the views. Sometimes it can be cloudy but dry with good visibility; and of course often it's glorious!