First off, i'd add Paragraphs if i were you as it gets a bit confusing reading it all in 1 sentence lol
If she's pulled her Feet away from you, then she's probably not comfortable with it, so maybe i wouldn't tell her anything. I mean, why would you want to anyways? Sounds like she'd find it disgusting.
You think she may change her mind and like it if you told her?
Up to you but maybe certain things are best left unsaid.

You can tell her, but be prepared for a response you won't like.  She will most likely tell you that it's not appropriate.  It's good to get to tell her so that you can get the urge to tell her out of your system.  Your challenge after that will be dealing with her reaction.  You will need to figure out what to do with your disappointment, which is healthy.  I like the suggestion given of you then moving on to focus your attraction for feet to a girlfriend.  This may help you get over the desire you have for your mother's feet, and I feel would be the healthiest, most natural way to solve your issue.  Lots of kids when they are younger at first have attractions to family members as they are growing up and figuring out what to do with their sexuality.  Eventually most grow out of it, or "work" through it.