I don't really know anything about the "dot" kind of psoriasis.
If it's on the area where you want to be tattooed, I'd wait for it to clear up first,
or find a clear area to be tattooed on. They probably wouldn't tattoo on top of
the psoriasis anyway. If the dots are anything like the scales, the tattoo might
not stay any place where you get a breakout later. That is to say, your tat might
end up with clear or faded spots in it, if some dots pop up there after you get it.
I think if you pick a spot that always stays clear, I think you should be okay.

Anyway, that's basically what I was told when I was thinking about getting a tattoo.
Maybe someone more knowedgable will pop up with more info.