I'm afraid I don't know much about Oscar Wilde, but if I'm taking that quote in the correct context than I understand where he's coming from, but in the wider scope of things I respectfully disagree. It is very much a possibility for someone to spend too much time on themselves, understanding their own needs and desires; while that can produce a shallow personality, it only does so if the person in question wishes or allows it to.
I have been trying to understand my mind, who I am and my place in the world since I truly became aware of my existence in the scope of things. While placing boundaries on yourself based on your assumed capabilities is a path to disaster, knowing yourself can bring contentment in the oddest of ways.
The foremost example in my mind is trust. Thus far I know so little about myself, but I trust myself to make choices for the benefit of others and for my own sake. An example would be when I plan to go out for the day; I have a dreadful memory, so attempting to find my keys may seem like a futile activity. As it stands however, I know myself enough to realize that the first place I look will often be the correct one, simply because my mind was in a similar state the night before when I set the keys down. It is akin to going on trips - when I've arrived at my destination I will think I've forgotten to pack something, only to find it is already in my bag because the week before I knew I would forget to pack it last minute, and so packed it then to prevent trouble for myself later.
It is... difficult to explain without perhaps sounding foolish, but as it stands I've been managing to overcome my memory problems by trusting myself, and that my past self is working for the benefit of my future self.
And in the words of Richard Kirsten Daiensai,
"The first step in understanding is the realization that you know nothing".