There's a lot to unpack here, but you capture it well. Some players get a pass. There are many examples in the chat of how people will go out of their way to defend a player they like because he's gritty, gutty and whatnot, and others who will seize on the one flaw for a player they don't like for some reason. Fowler may never be able to overcome his 2018 with some people. That's the nature of the business. He could go out and it a career amount of homers and have his career OBP and that won't be enough for the folks who somehow want him to be more than he was a Cub because the Cardinals are paying him more than the Cubs did, or something.
I am always reminded of Juan Encarnacion. He was a solid player for the Cardinals, exactly as advertised. And yet he was the subject of such ridicule from a corner of the fan base because he glided, he coasted. He didn't churn up the turf, like say an Aaron Miles or Abe Nunez. Now, Encarnacion covered more ground than they did, got their faster than they did, and yet how he looked doing it didn't satisfy because he did it with such ease. That always seemed silly to me.
And we've be naive not to note that some of the comments -- ones you can find on social media and I can find in my email -- have a race element to them. At best, that's unfortunate. At worst, it can be downright vile.