The effect of flap on the wing's various forces is always to make the wing pitch nose down, taking the rest of the aircraft with it. Any wing with camber has a nose down pitching force related to the amount of camber, lowering flap increases the camber and therefore increases the nose down force.
The primary reason there is a nose down pitching force is due to the net lift vector of the wing being behind the CG. This results in the nose down rotation. The horizontal stab's lift vector is a downward force and negates this rotation during straight and level flight. When flaps are deployed there will usually (depends on flap type) be an increase in lift due to the camber change, however the lift increase doesn't always result in a more pronounced nose down rotation because of how the tailplane interacts with the downwash from the wing. As you've mentioned, the wing downwash can be substantially higher than the nose down rotation caused by the increase in the wings lift. The net result is the nose up pitch as you've mentioned in your C-150 example.
Low wing high tails tend to have a nose down effect as the tail has little or no flow in the downwash.
This correlates to the concept of how the wing's lift vector causes a nose down rotation. This results in the need to increase the downforce the tail provides, or nose up trim.
Rich