When Mayhew first came to the Iowa Wild as a college free agent he wasn't getting Top 6 minutes. The emerged over time as near the end of that first season he went on a bit of hot streak showing some of that scoring potential. Then performed well in a go-to role last year.
Should he be on the team? Sure, but @IllaZilla hit the nail on the head why he isn't. It isn't anything he did wrong and his presence won't matter much because of the limited role he'd get with the big club.