There are now two KLM Crown Lounges at Schiphol, Lounge 25 in the Schengen Zone and Lounge 52 in the non-Schengen zone.

Both of them have showers, which are (in my view) a bit nicer (if a bit smaller) than the ones in the Mercure. They also provide you with amenities (shampoo, shaving cream, razor, comb, toothpaste, toothbrush). They are also often very busy. The new Lounge 52 has ten showers, and I've waited well over an hour for one before.

I haven't used the Mercure showers in ages. That said, I recently stayed in the airside hotel again (first time in several years) and the rooms have been remodeled and are much nicer, so maybe they remodeled the showers too?

Hope this helps.