I believe that ASAP Yams was one of the most underrated figures in hip-hop music because of his role in building the careers of other members of the ASAP Collective, such as ASAP Rocky. The massive amount of influence that he had on the hip-hop world at such a young age is rarely seen in any genre of music.
While Yams contribution to hip-hop is pretty decent with his involvement in ASAP MOB, he was not one of the most underrated figures in hip-hop. He was pretty much a promoter, that's the end of it. No writing credits, no production credits, just a taste maker so to speak. Asap Mob is no Wu-Tang Clan.
ASAP Yams was not underrated. He contributed and was recognized for what he did. He simply did not have as much popularity as some other hip-hop figures. His national success was not so apparent to American audiences nationwide, but he had his niche followers, which often happens in every circle of music.
You know, it's a shame about his passing but I don't see him as being "underrated" at any level. Not everyone in the entertainment industry is going to go have worldwide fame and riches. But he did have industry respect and influence, and that's what counts. Based on that, I think he did just fine in terms of rating.