Should Congress investigate the conman's practice of pardoning military personnel convicted of war crimes as possible grounds for impeachment?

Pardoning them was wrong — war crimes call for punishment, and the US usually fails to try the perpetrators.

Whether a politically controversial pardon should be grounds for impeachment is a different question. I think a president ought to pardon whistleblowers and journalists, including Edward Snowden and Julian Assange. That too is politically controversial. Should a president be impeached for pardoning them?

Should Governor Altgeld have been impeached for pardoning the union leaders wrongly convicted of the Haymarket bombing? (Their "trial" didn't try to prove they had anything to do with it.) He was quite unpopular after pardoning them, but the act was not treated a crime.