D&D 5E - Monk's Slow Fall and getting knocked into a wall...

The Monk's slow fall relies on the ability to mitigate the speed of the impact by using the surface to slow you down.

Even if you decided that slamming into the wall causes additional damage, I would deny the Monk any mitigation effect based on this unless she was capable of moving Parallel and within touching distance for at least 50% of the "fall".