Does a plane have more lift while flying in a cloud? (All other things equal...)
No, higher humidity air is less dense than lower humidity air.  This is because water molecules are lighter than the nitrogen and oxygen they would replace in an airmass.

https://science.howstuffworks.com/nature/climate-weather/atmospheric/is-humid-air-heavier-than-dry-air.htm


  any water vapor that gets added replaces either nitrogen or oxygen in our free-moving air. Nitrogen and oxygen make up the majority of our atmosphere, and they're displaced -- or evaporated -- when water takes their place in the air. And water vapor molecules are lighter than both nitrogen and oxygen. In other words, humid air is going to have less heavy nitrogen and oxygen -- and lighter hydrogen and oxygen -- in its place. Remember that they have the same number of molecules, but the air with water vapor is simply less dense 


Humidity, therefore, is also an important factor in density altitude calculations.