My DD would fiddle with my mouth/lips and then she moved onto poking her finger into my belly button and touching my mole on my tummy. Which would occasionally get rough!

I would hold her hand and stroke it, taking it away when she did so. Or, I would let her keep touching me, but only if she was gentle. You can be firm/assertive and still give her the comfort of fiddling. Telling her she can only touch somewhere on Mummy (if not nipple, pick something else!) if she is gentle, otherwise she will hurt Mummy. It requires constant reinforcement though! My DD did this for years, but at least she understood the gentle thing relatively quickly. I'd use the dummy and redirect her touching, encouraging her to be gentle.

I completely sympathise with the ouchiness!