Jackie Robinson put up good numbers for bieng a second baseman, a weak hitting position, and then when you consider the numbers he had in the Negro leagues, he was a great player...not a mediocre one. You have to think of the era in which he played, not relate the stats to today's inflated offensive stats.
I dont think he's overrated, but I think people forget how good a player jackie actually was, and consider his "breaking of the color barrier" his only reason for being in Cooperstown, but thats not the case.
Random piece of trivia for you, did you know that Jackie was not the first african american in baseball however. Fleet Walker was.