"Given the underperformance" implies that people had the right to expect more from the team Stearns assembled. There is plenty to debate there, but guys not playing to realistic expectations are not the GM's problem. Guys getting hurt aren't necessarily the GM's problem either, unless they came in with an injury history (or are pitchers) and were paid like full seasons could be taken for granted (see Polanco, Jorge). As far as the "excessive reliance on spreadsheets" goes, anybody who can read a Fangraphs page knew Peralta got lucky last year.
There's a lot of anger over dismantling the team's core (Alonso especially), but I get the anger and I get concluding the Mets had gotten as far as they could go with them to equal degrees. Like I said over the winter, I think Stearns would have been forgiven if he put a better team on the field, but he... didn't.
What I didn't (and most likely won't) forgive Stearns for was the way he completely botched the deadline last year. Bringing in Mullins actually made the team worse. There were available relievers having better years than Helsley that would have cost less. (And Stearns would have known that if he relied excessively on spreadsheets.) And he didn't get any righthanded bats, which they needed. So I'm with ashie as far as Stearns handling this year's deadline goes.
That being said, the mess we have is not all Stearns' fault. The team has gotten some crummy luck with injuries. Soto wasn't out for long, nor did it compromise his play for an extended period of time, but the season was already over by the time he got back. Lindor has just not been right. I don't think I appreciated how brutal losing Clay Holmes would be, but given how our other veteran starters have performed, it's been brutal. But we have a new young core emerging, some players will bounce back, and we'll have money to spend. Next year is not a lost cause, and the deadline needs to be approached that way.