Well, part of the reason that the Feds didn't take the Mafia seriously for a long time is because they didn't believe the Mafia was as big an enterprise as it was. The Mafia was able to fly under the radar for a long time because of how they did their business. So you're basically saying they are overrated because they did a good job. That doesn't make sense.
Either way, it's not really relevant. The question was why do these rappers seem to idolize the mafia. Those rappers weren't doing research on how successful the Mafia actually was. They just saw that the Mafia is pretty much synonymous with organized crime at a high level in this country. And the point La was making is that if you're a criminal with aspirations of being the best, you're going to be inspired by the criminals that are portrayed as the best.