He gives it as his opinion, that the controversy between Jews and Christians is a most foolish one, and asserts that the discussions which we have with each other regarding Christ differ in no respect from what is called in the proverb, 'a fight about the shadow of an ass.' and thinks that there is nothing of importance in the investigations of the Jews and Christians: for both believe that it was predicted by the Divine Spirit that one was to come as a Saviour to the human race, but do not yet agree on the point whether the person predicted has actually come or not.
http://www.newadvent.org/fathers/04163.htm
The Alexamenos graffito shows Jesus himself as an Ass.
In the Pagan mythology the Ass is symbol of evil and destruction.
So it is not coincidential the fact that Celsus identifies the intra-Jewish polemic about the Christ as a discussion about ''the shadow of an ass''.
So I think that the same historicist belief of a pagan as Celsus is in the same his anti-Christian interest: one despises better Jesus if he reduces him to a mere man, to a sub-human, to a degrading feature of the nature, an Ass.