Is the kite considered a motif in The Kite Runner?
A motif is a recurring symbol or element in a work, and Hosseini does rely upon kites as one of his motifs in The Kite Runner.
To identify the kite as a motif, notice the appearance of it throughout the novel. The first literal kite is the one Baba purchases for Amir for the annual kite contest. The second significant kite is the one that Hassan runs down for Amir (the last fallen kite and prize of the contest) and for which Amir betrays Hassan. Finally, at the novel's conclusion, Amir runs after a kite for Hassan's son Sohrab. So, if you are writing a justification for the kite as a motif, you can certainly prove that there is a recurrence of kites.
After you establish an element as a motif, ask what that motif means. Why would Hosseini choose kites specifically? For the Afghan culture, kites are a tradition, and Amir is willing to sacrifice everything--even his friend's well being--to obtain a "paper-thin" connection with Baba because Baba places a great deal of importance on Afghan traditions and his culture's penchant for winning. The kite motif serves to advance the theme of winning in the novel. Amir discovers through kites what winning can cost someone, that winning is not permanent, and that winning is not satisfying if one has nobody to share it with.