Loads mate. I've never held court in the oval office but I've sat in boardrooms with CEO's that have put their feet on the table, women have taken their shoes off, guys have got rid of their ties, I've seen others spin the chair around and sit on it backwards. In media companies the CEO has worn flip-flops, the FD has worn a beanie. At an auditing firm the meeting chair has sat on a swiss ball. I could go on forever. Times have changed.
Ignoring all of that the photo in question wasn't a meeting, she's in the background probably doing some work and then took a photo. I doubt she was on twitter. It's a storm in a teacup and people getting p1ssy over her feet being on the couch are as bad as Trump in many ways. It's a picky non-issue.