I wouldn't say zoro is underhyped at all. Certainly not compared to sanji, marco for example. I think all the M3 have comparable strength, however luffy has a goku like ability to become a lot stronger in some situations ( the others do too but not to the same extent ). For example luffy fought ceasar at punk hazard and didn't look impressive, but now he is going to take on and beat DD. Zoro and sanji do similar things but not to the same extremes. That's not to say that they can't, it's just we haven't really seen it happen post ts. Sanji taking on big mams crew will be an example ( but I don't think there will be a full on fight as I think lola will be revealed as the daughter of big mam ), zoro vs fujitora would be a good test but that was a very brief exchange.