How thick a layer of straw, and did you pack it down?

Mulch should always be added when it is dry and light and fluffy, sprinkled, not added in clumps.

Sure, if you feel it was too wet, then remove the wet mulch. I'm sure the straw on top is dry, so skim that off first, set it aside, take off the wet mulch, and lightly sprinkle the dry straw you have left around the stems.

Maybe they are finished? Although it seems a little early for that, but you never know. When did you plant them? Pull up a few and see how they are doing.

BTW, la Ratte is an excellent potato.

Good Luck