Underrated and overrated are such subjective terms. If you don't think someone is the G.O.A.T. or the MVP that doesn't mean you are underrating them. And if you don't think a player should be waived, that doesn't mean you are overratting him. For example a player like Marcus Smart was overrated by many Celtics fans, while also being underrated by other Celtics fans. He was neither an All-Star, nor the cause of the team's losses.
Now Paul Pierce due to remaining in the public's eye since his retirement has done a ton of boasting of himself, which has drawn the ire of non-Celtics fans. Add in the fact that the general NBA fan tends to have a "Boston sucks" attitude and you could see how a new generation of fans might underrate Paul.
Hopefully it's not coming from Celtics fans, but who knows. In the latest episode of Bill Simmons' podcast he discusses with Austin Rivers how Pierce is viewed. If you recall, Austin spent a good deal of time around Pierce since his dad Doc was Boston's coach for much of his adolescence.