I have tried to estimate this before and links are provided below. If you read them and click on links to the complete rankings, Murcer does not appear to have been underrated.
Here I just looked at where he ranked in WAR among position players and in MVP shares among position players. Here are the WAR leaders from the 1970s using the Baseball Reference Play Index
WAR leaders of the 1970s.
Murcer is 38th.
I also copied and pasted all the MVP votes from 1970-1979 from both leagues into Excel and found the cumulative total for each player. I took out any pitchers. Murcer is 36th, very close to his rank in WAR. So it seems that he was rated just about right by the MVP voters. He had three top 10 finishes (a 7th, a 5th and a 9th).
Was Willie Mays The Most Underrated Player In History? Or Was It Wade Boggs? Is Albert Pujols The Most Overrated? (Revised)
Using A Player's WAR To Predict First Year Hall Of Fame Vote Percentage (and possibly estimate "underratedness")