Definitely highly inappropriate, and this is coming from a guy with a slight foot fetish. This isn't your house, or apartment, etc. Everyone paid for the flight and people didn't pay to have your bare feet around them. ESPECIALLY if they're dirty or nasty or stinky.
The fact that people are STILL doing this despite the widespread media attention this issue has gotten over the years is beyond ridiculous. If there were ever a case in point example to prove humans were selfish, thoughtless creatures, this would be one great example