People who are "mad" at this are actually just generally pointing out the injustice of a poorly reviewed and fundamentally flawed movie like Suicide Squad winning an Oscar when more deserving and beloved movies and directors have not.
But I think most people understand that, yeah, for makeup and hairstyle, sure, Suicide Squad had a really good chance in that category. It doesn't actually speak for the quality of the movie itself though.
While I understand and agree with everything you said, a part of me is still glad that the people who probably dedicated a lot of time and effort into a promising movie that flunked for dumb creative decisions script and editing-wise were able to get one bright spot in the aftermath of this experience. Which maybe even help them work on movies that, you know, are actually good and deserving of their talents.