How is he underrated? You have to elaborate a bit more on that. If by underrated you mean that he is not considered in the same breath as the modern masters Ponting, Dravid, Tendulkar, Kallis etc...then he is rated perfectly, he is not just not as good as them as I have been saying quite a lot on this forum lately.
Infact, him breaking the record of Viv Richards and scoring a truck load of runs on flat sub continent wickets has to an extent clouded some people's judgement and they at times overrate him.
He is not a bad player by any means, anyone with an average above 50 and over 20 test centuries cannot be a bad player, but to differentiate the very best from the very good, there are certain things you look at and Yousuf does not fare very well then.
Lara, Tendulkar, Ponting, Dravid have all thrived against the best bowling attacks of their times, and won/saved matches at times. Ponting' 156 at Old Trafford comes to mind, Dravid's 230 at Adelaide comes to mind, Lara's 200 against Australia in 99 comes to mind, Tendulkar's 241 in Sydney comes to mind. Yousuf's below average record puts him well below those 4. I dont give too much importance to matchwinning centuries becaues I believe it is often beyond one player to win a test match, so I wont hold that against him if he was phenomenal against South Africa, Australia in losing causes. In 2006, when he was at his very best, he only managed a 2 half centuries in South Africa.
See scoring 80s, 70s in trying conditions against a good attack is a very good effort, but its very natural we will rate someone who scores a 150 in trying conditions against a good attack higher.
This current series is a great example. I can understand that Pakistan are fielding a very weak side, with very weak openers, but thats no excuse for Yousuf's failures. He failed miserably in the first test, when a greater player would perhaps have scored a century in the fourth innings and won the match, in this match, he should have gone on and scored a century when he had a start, but only managed 80 odd. Once again, thats a very good effort, but that wont make you a modern master, just a very good batsman.