As far as I know, the Church has never condemned barefeet.
It is very difficult sometimes to draw a line and separate what is feminine and what is immodest. There is a question of culture and custom too.
I have a liking female feet, but, for me at least, ugly sandals won't make me uninterested. Nice feet are nice feet, no matter the woman is wearing.
Feet are not considered indecent, as you can see on every Moral Theology manual.
If you have a particular attraction to them, you need to find a way and work around it.
Women are usually not wearing indecent clothes at mass, so, you might be safer looking up than down. It is not like they are wearing bikinis and the only safe place to look is the ground.