Any implementation is of course free to use a reputation system, but it seems hard to implement. You don't necessarily know all the votes a remote user has received. Say you get a vote to a post from a user who you've never heard about before. But actually this user is a well-respected member of their own instance and has been on that instance for years. Meanwhile, your instance believes this is an inactive spammer or new account or something.

Who determines the quality of one's posts though?

The users? Users are reactionary and often vote based on how a post influenced their feelings. It probably works on Stack Exchange because the scope of the forum is solving technical problems.

This is actually something I have not thought of, the only issue is that people are going to use it as a I like this instead of this is high quality, which I think is the biggest hindrance of that. This is also going to be nearly impossible to moderate on a federation level because an instance could be spun up that would lie about the reputation of an account and everyone would just go to that instance due to the fact that it doesn't have that restriction

-1 for rep/karma system. Most people agree that it's good we don't have karma here.