A nosy neighbor is okay if one is not always spying and prying into the lives of one's neighbors because we all have the right to live as we choose. But keeping an eye open for unusual occurrences helps protect that neighbor against theft and also might protect someone in the house from domestic abuse.
There are limits of course to what measures may be considered extreme, but being a "nosy neighbor" keeps a person well informed of the basic happenings in the persons immediate environment. Knowing who your neighbors are and their habits may prevent robberies or other security threats. Knowing if someone in the area looks out of place could be an indication of a threat.
It is not okay to be a nosy neighbor. It can harm both the person being watched and the watcher. The person being watched can easily have their privacy violated. The watcher can also be pychologically hurt by investing their time into the private aspects of other poeple's lives. They might find out things they don't want to know and have to live with that information.
I firmly believe that man is a social creature, and as such, a strong sense of solidarity with one's neighbors is a vital aspect of our social and cultural development. However, friendliness and familiarity goes only so far. While I believe it is healthy for one to take a vested interest in the lives of one's neighbors, there are still boundaries.
No, it's not okay to be a nosy neighbor. Neighbors should mind their business unless something illegal is going on. People deserve their privacy and shouldn't have neighbors peeking out the window or listening though walls trying to find out what is happening in their lives. Nosy neighbors are the worst.