Foam I would say is messy as it's sticky and has a visual effect. Oil and some slime, especially thin and/or uncoloured, appears closer to wetlook.
Shampoo itself I would say is a gunge as it's made with very similar ingredients (xyz-cellulose) BUT isn't messy as it's used for cleaning! In the same way someone putting on a face mask as a beauty product is not WAM in my mind. It's also again not generally coloured so you don't see the effect.
I put it as 'somewhere in between' as I'm too indecisive to choose.
Shampooing (with soap) is one of the common obsessions that (I've observed) tend to coexist as "sub-kinks" with WAM along with other examples like bare feet, playing with long hair, smoking etc.
It is highly relevant and appropriate, though, as, although it's literally a cleaning agent being used in a sensualised, performative process of cleaning, it creates a foam. Foams like shaving foam are widely accepted as WAM (whether many of us like it or not!)
Conventional soap shampooing doesn't do a lot for me, personally, as the entire focus of a piece of media and neither does any kind of foam.
(Unless, of course, there's a contextual angle like, say, it's carried out on a fully clothed volunteer in the street wearing an unprotected suit. Then it's matter sufficiently out of context to create the necessary anarchic chaos.)
Its conditional, no pun intended. A shampoo can be done in a way that activates the wam neural pathways, especially if the shampooing is done messily, with facial coverage or using a non shampoo substance.