When Maniac crossed the finish line backwards, why was Mars Bar embarrassed in Maniac Magee?
I believe that you meant to ask why Mars Bar was embarrassed when Maniac crossed the finish line backwards. I have changed your question to show this.
Mars Bar was embarrassed when Maniac crossed the finish line backwards because it meant that Maniac had beaten him really badly. Mars Bar, who had racist feelings towards Maniac, had challenged him to a race. He felt sure that he could beat Maniac. But then Maniac beat him by such a great distance that he was able to turn around (without meaning to) and cross the finish line backwards. This was embarrassing for Mars Bar because he lost the race badly in front of all of his friends.