Well, technically the street is public property, so as long as it's not a "no parking" zone, it's not wrong.
It is a little insensitive though. I mean, generally I would park my car in front of my own house, not someone else's, unless all the space in front of my own house was taken.
If the neighbor being parked there was preventing me from getting my mail, I'd call and ask them to move thier car, and explain why.