NPR already airs opposing viewpoints regularly. I've heard them interview Republican senators and conservative commentators many times this year right alongside with people on the other side of the aisle.
If you mean NPR needs to carve out some time dedicated to a particular political view, then perhaps your perception of them is the rest of the time they present an opposite view? In these highly politicized times perhaps it seems that way since people have trouble agreeing on things that are widely accepted and based on science, as one example. So any news that presents that as fact will naturally be dismissed as biased by anyone who disagrees. Personally I don't think it's really incumbent upon NPR to create more air time for that kind of disagreement; they should stick with the facts. People who disagree with fact-based journalism already have many other outlets and are (IHMO) disinclined to seek out anything that will challenge their views.
Even so I daresay if someone is on a journey to find and listen to opposing viewpoints, the person who finds NPR to be a bastion of liberal values would be far more challenged by listening to Pacifica. If someone wants to really hear viewpoints left of center, they should try listening to Democracy Now for a week.