I think it is reasonable and justified to poke fun at Apple for this. This is the company that prides itself and presents itself as taking great pains to pay attention to detail... spending 9 hours to make the stainless steel wristband... filming a jellyfish at 500fps at a resolution of 9000x9000 pixels so that its motion can be captured perfectly... and so on.
To not have a disclaimer regarding tattoos indicates that they were caught unaware. That's the type of attention to detail that would've been covered.
Having said that, any disparaging remarks about those with tattoos, even in jest, is pathetic.
Maybe it works with most tattoos, and only a small amount of them don't work. Your assuming a lot here. Seemingly, even with people with no tattoos, it can fail; the condition for it failing must be incredibly varied, but rare, or Apple would detail them on their side.
I myself have four tattoos and if I had one on the wrist that made my watch not work properly, I wouldn't whining like a baby about it.