Re: In today's news...
Microaggression is, sociologically speaking, just a term for a minor annoyance that is repeated so often and by so many that at some point you just snap.
Now, I'm not going to answer for how people use the term in activism, because there are a lot of idiots in the world, but the term in itself is not bullshit, it was created to describe a particular phenomenon for which there wasn't really a good, concise term previously.
It's debatable whether "aggression" was the right choice of words, because a lot of examples rely more on people being clueless and tactless, but that's down to semantics.