It's normal to feel upset when Napoli doesn't play well all the time. But every team has good and bad times during a season. It's not fair to give up on them just because they haven't done well in a few games. Napoli has been winning lately which shows they can do better and compete at a high level. We should support them when they are doing well and when they are not because that is what real fans do. Let's hope Napoli keeps winning and shows everyone they can do it. 
Well, if we are fans of a football club then we must always support them when they are good or bad. Likewise if we become Napoli fans or other clubs. because it is true as you said that a club cannot always be at their best. sometimes they can also have a bad season. But that doesn't mean they will always be a bad team. And what happened to Napoli was that they had a quite drastic decline in performance. So it would be very natural if their fans also felt disappointed. But now, after changing to a new coach, it turns out that Napoli is showing signs of improving performance again. So Napoli fans will definitely be very happy to see this.