People don't rate rappers within the same criterion. For some, technical aptitude may be most important. For others, the ability to touch your emotions and stimulate intellect may be more important. And there are people who value a balance of both.
So I don't believe in the terms "overrated" and "underrated" when it comes to rap because people aren't rating with the same scale.
That said...if you UNDERSTAND each scale, it shouldn't be hard to understand why millions of people consider Tupac to be one of the all time greats and/or arguably the greatest.