I agree with the others especially if you plan to include the sky, but a landscape does not have to include the sky. An overcast day helps to control the shadows in the shot. If the clouds are interesting, then make sure they are a prominent part of the shot. However, if the clouds are boring and just a dull gray, then you compose so you show very little or none of the sky. Ugly overcast just means you switch philosophies, and maybe zoom in and isolate an aspect of the landscape. Waterfalls are great on overcast days by keeping the light more even and diffused, and you do not get that washed out areas were the sun reflects off the white water. If you can get fog or mist, then all the better (in fact, I look for those days to go shooting).

Here is an example of when it was a dreary day at noon to mid-afternoon, but the atmosphere made the shot. In this case you make sure the foreground is interesting and it lets you get the idea of what the rest of the background looks like. Also notice I only showed a small amount of the sky so you get a faint hint of the horizon on a rolling hillside.