1) The definition of "shitposter" and "shitposting" are too vague.
2) OP's bias would always factor in, which could stifle legitimate conversation.
3) What would OP do after all of the mutes are used up? What should have been done from the beginning: report the posts.
4) It would lead to inconsistencies in moderation. One poster might be muted for a post and another might not be for the same post simply because the OPs had different opinions. A side-effect of 1).