How is lift provided by a wing affected by propeller wake?
It both increases and decreases, depending on the local direction of the propellor (e.g. on the upgoing side, or the downgoing side) . See for example this PhD thesis: Propeller Wing Aerodynamic Interference. 

This thesis shows the following image:


Simply said, the velocity of the propellor locally changes the angle of attack, and thus the lift generated by the wing.
It is interesting to know that it is possible to locally optimize the shape of the wing to take advantage of this effect. Again the thesis shows the result, and it looks like this:



If you look closely, you can see that the wing is changed at the propellor location to accomodate the adjusted flow.