It is a tough spot to be in, because on one hand your husband is already managing a strained relationship with his mum over feeding issues, and on the other hand your concern about hygiene and physical boundaries with baby is completely valid too. I think the real issue here is less about the smelling itself and more about feeling unsupported, so it might help to first have a calm conversation with your husband about that specifically, separate from the MIL topic, so he understands you are not asking him to go to war with his mum again but just need him to acknowledge your feelings. If you do decide to speak to MIL yourself, framing it gently around baby's sensitive skin or health rather than "it's disgusting" tends to land better and keeps things less personal. It is a balancing act lah, but sometimes going directly to MIL in a soft way actually works out better than going through the husband, especially when the husband is already stretched thin on that front.