you are right that it would be better hide such gestures, but on the other hand he is also just a human being and we tend to let our feelings decide what to do.
i don't think that diego is happy that we lost just to make ferrara look shit. deep inside him he will be probably be angry with ferrara and would like to go to the coach and tell him his opinion, but as player you are not in the position to do so if you want to keep playing football

the smile is probably really a way of overplaying the angryness he feels about the ones he thinks are responsible for this weak performance.
you know even alex sometimes has an emotional outburst (just remember the throwing of the jacket he got handed, after being substituted

)
i just hope i wake up tomorrow and read that ferrara is not in charge anymore ...