I'm not sure Robert, Semien & Mauricio are significantly underforming expectations. At least, not mine, as I really didn't expect much out of them. Semien and Robert have been in decline for a few years going into this season. To count on the two of them to suddenly rebound and be significant contributors was (and is) folly. Mauricio, has never really impressed, so my expectations were quite low.
Baty and Vientos have regressed and I don't expect either of them to ever be dependable regulars. Vientos in particular, I am over. He strikes out too much, walks too infrequently so his BA & OBP are pathetic and not conducive to a productive hitter. Then to further reduce his value, he is a defensive liability at any position they have played him.
Bichette is the player that has grossly underperformed my expectations. He has at least been better the past month or so, but it's too little too late. As disappointing as he has been on the season as a whole he leads the team in RBIs.
Lindor always starts slow, which is problematic, but he generally turns it around as the weather warms. Unfortunately, his injury derailed his season.
I really didn't know what to expect from Polanco. I think the Mets counted on him to remain healthy and replicate his 2025 season while learning a new position. Considering 2025 was one of his best seasons both in production and health, it was a questionable plan at best.
Okay, I get off my soapbox now.