Yes, a continuously loud dog that has received lots of notes from the neighbors should definitely be considered harassment. There's no doubt about this. When you have neighbors, you need to be respectful toward them regardless. There's no doubt in my mind that it is wrong to allow your dog to be loud despite several notes from the people you live near.
Daily notes about a loud dog would not constitute neighbor harassment. The neighbor is letting the other neighbor know that the dog is very distracting to them and hoping that by sending them a note hoping the neighbor will fix the problem without further action. The person with the dog should correct the problem so they can all live peacefully. Its the polite thing to do.
No, daily notes about a loud dog would not constitute neighbor harassment, because the dog might be every bit the problem that the neighbor says it is. If the person is justified, there is no reason that the neighbor can't let the dog owner know how annoying their dog is being.
Different areas have different laws regarding loud and disruptive animals. A loud barking dog, day in and day out can become quite annoying and dog owners should be aware of this problem and stop it from happening. I think it is improper to address the issue with daily notes, but I wouldn't consider it harassment. Respect your neighbors, don't terrorize their at home time with your noisy, loud, barking dog.