One of the reasons why strong, vocal disability advocates get more negative than positive response in social media ... such as when they criticize Inspiration Porn ... is that most people find strong, critical, negative opinions on any subject to be unattractive. Advocates can shape attitudes over time. They can bring about important policy change. A significant minority of people actually admire advocates and love what they do. But on just about any issue you can name, advocates and social critics who speak their minds are rarely liked.
This phenomenon is a bit more intense and hypocritical in the disability sphere because of the unique characteristics of ableism. But I don't think the backlash is much worse or all that different from the responses people get when they express challenging opinions on race, gender, politics, religion, economics, etc.
Well, it suggests that if you're going to be an advocate, especially in the realm of disability, don't be surprised if you catch a lot of crap for it. If you're very good at it, and articulate, you might gain a small but loyal fan base within the activist community. If you're smart about strategy and don't take things too personally, you can succeed in what you set out to do. But if you venture out into the wider public discourse, don't expect to be either liked or admired. Change is uncomfortable. Most people don't like to be uncomfortable. And people absolutely hate it when the people they think they are helping are the ones making them feel uncomfortable.