Pilot reported encountering downdrafts in icing conditions resulted in loss of thrust and an uncommanded descent. After reaching a lower altitude where VFR conditions prevailed; control was regained and the flight continued safely.

Narrative

Took off IFR from ZZZ at XA:36 with destination of ZZZ1. Flew SSE to cruise climb and avoid cloud building and reported turbulence over the [mountains]. Received clearance to fly VFR on top in order to have directional and altitude autonomy as we approached the [mountains]. I was originally going to go south of Lake and I noticed that I had what appeared to be a clear shot of lower-level clouds to cross the [mountains] toward a passage area. I began the path as the windward clouds seemed to be ascending with us as we also began to feel increasing light up to moderate turbulence. I kept our climb toward my goal of 14;000 ft when I experienced a sudden downdraft that I resisted as our climb and airspeed began to dwindle. We began to losing altitude and entered into the top of the cloud below us around XB:20. I immediately turned right toward lower terrain anticipating break-out toward apparent earlier clearing; but we continued to descend while picking up immediate rime ice. I notified [approach of] our icing dilemma the windshield immediately occluded building to about a quarter inch over the leading wing edge as we also began losing prop thrust. I was instructed to turn left and attempted but my attitude indicators were in disagreement; secondary showing left 25 and primary showing 35-degree left bank. I began descending to avoid stalling and informed [approach] that I needed to concentrate on keeping wings level as we dropped over two thousand ft. and broke back out into the clear. I maneuvered to stay clear of clouds and terrain tops as I arrested the descent; regained airspeed; and ice began to slowly diminish from all surfaces and we progressed on to ZZZ1. I commend ATC for advisories and assistance. I admonish myself for taking the 'sucker hole' clearing over the valley to avoid the restricted areas. My 'do overs' would be the following: Lighter on the fuel weight; further southern crossing and get my AI looked at.