There are several synonyms for the phrase "more underachieving". One such synonym is "less accomplished", which refers to someone who has not achieved as much as they could have or should have. Another synonym is "falling short", which suggests that someone is not meeting their full potential or failing to meet expectations. "Unsuccessful" is another synonym, which refers to someone who has not achieved their goals or objectives. "Underskilled" is another synonym, suggesting that someone lacks the necessary skills or abilities to succeed. Ultimately, all of these synonyms suggest that someone is not achieving as much as they could be.