She’s been getting a lot of bad press on social media and alt media regarding her health. She wouldn’t add fuel to that fire by faking her illness IMO.
She's not faking. But in the spirit of "never let a good crisis go to waste," I think the pneumonia diagnosis, which I don't believe given her history, is a perfect cover for whatever more serious chronic illness she has.
Pneumonia can be cured, but perhaps not fast enough for her to be in the debates, which gives her a good excuse to miss them, without it reflecting on her general health issues.