I once faked interest in a girl's foot injury just to see her feet
Why did you fake interest in her injured foot?
Because I've been wondering how her feet look
It sounds like you might have a foot fetish. It's considered by most rather creepy to satisfy your sexual urges by being deceitful. You should keep these things to yourself.
It's okay, she did not notice.
But you have stated your ulterior motive and I do not think that it is okay. I do not think you would like it if someone would sexualize and objectify you constantly in the most inappropriate way. If you want to help people with injuries, make sure you help them for a good cause, not secretly have sexual intentions with them.