TBH, numerical ratings are very subjective and are a silly thing to get upset over anyway. A perfect 10 to someone might only be a 5 to someone else.
While I'm mixed on this cameo, I don't think it's going to ruin the game for me. I mean, we have other knights in the same level, so it seems like a fun and cheeky inclusion. Who knows, maybe it turns out that Shovel Knight is secretly a pig as well! It'd make sense in-universe for him, as animal people exist in his game.
Either way, I think people are mostly upset because this wasn't the "cool announcement" that they were hoping for.