Who is this man whose face appears randomly?
He is the mangaka's assistant, Maeda-kun. 

From the TVTropes page Manga: Sayonara, Zetsubou-Sensei:


  Fan Disservice: Some fanservice scenes can be a little awkward when they're censored by the face of Maeda-kun, Koji Kumeta's assistant, who sometimes is even facepalming disapprovingly. Shame on you.