In the Marvel Comics, one of the Gene Nation members is simply known as Sack, a mutant who can cover an opponent with his transparent, gel-like body and make them fight for him, due to his body being very weak. I am curious: has he ever used this ability to try and suffocate someone? After all, if their face is covered in a thick, gel-substance, I imagine it would be very hard to breathe.
The very short answer to this one is no. Sack's mutation isn't that he's strictly a liquid, but rather that he can turn into a sort of ethereal energy and merge with another being, allowing him to operate their body like a meat-puppet.
That being said, the process of absorption is 100% fatal to anyone who doesn't have a mutant healing factor.