I dont think Imran is underrated as a cricketer overall, though perhaps he is not put the level he deserves. I would consider him one of the 5 best ever. He gets selected in many all-time XIs, though one reason he isnt in more is because they aribtrarily limit themselves to only 1 all-rounder per team, giving the space to Sobers.
But, as I have said in the past, his captaincy and status as an all-rounder tend to overshadow his achievements with the ball. I would argue at his peak from 1980-83, when he had pace, skill and reverse/conventional swing mastered, he was the best fast bowler ever. It is still rated the highest peak by any post-WW2 bowler. And if you add in the fact that he was the first true paceman from the subcontinent, the first bowler to show the potential of reverse swing, and that unlike the overrated Warne, he actually left a bowling legacy that he singlehandedly inspired in Waqar, Wasim, Shoaib, etc, as a bowler alone Imran is right up there with any of the best, maybe a flicker below Hadlee/Marshall perhaps.