I’m in the camp of not booing. Agree that as paying customers people have the right, but as supporters, at some point we need to decide if we are going to try and be a 12th man for our team or the opposition’s. Confidence plays a big part in performance and some players are affected more than others. Wide players in general (and I think McMullan in particular) are “confidence” players, with more onus on them to “beat their man” than most others. They also play closer to the crowd than others and more easily pick up heckling etc.
It doesn’t even take actual booing, I think players pick up on the overall atmosphere including crowd groans and sighs.
When the team is playing well, supporters will ignore the odd poor piece of play, but when we go to the ground assuming we’ll be poor, it can start from the first stray pass and set the tone for the rest of the game.