It is for now, but its 512 RAM is very small and its CPU is very slow, so validating the blockchain alone takes several weeks.
It's feasible, and there are people running nodes this way... But obviously not on the 512MB RAM Pi B You've got to have a Pi B 2, which has 1GB RAM.
For now you can even run the node solely on the Pi, using a 128GB card. You can even run it portably, on an external battery, using wifi or a 4G connection (I haven't tried this, but it's not an impossibility)
Hence the "for now" I dont think it will be feasible in the future, but I might be wrong. 0.12 comes with a way to restrict memory usage, but I guess the CPU will be too weak once SegWit is introduced or at the very least once bigger blocks come. I also havent read anything good about using SD cards to store the blockchain, apparently they are worn out quickly.