A Michael Keane own goal and one from Sheffield Wednesday loanee Chris O'Grady were enough to ensure the Tykes climbed back out of the bottom three at the first attempt.
Veteran striker Jason Scotland also saw his second-half penalty saved by Kasper Schmeichel.
"We were way off the mark, the entirety of the first half was very poor," Pearson said.
"We remain a side that has too many players out of form at the moment. Collectively you can still negotiate these types of games but we're not doing that.
"I don't feel that we are making enough good decisions during the game. It was not a very good performance at all.
"We're at that stage of the season where we've worked hard to get into a position and over the last two months, not always through playing badly, but we've had too many days where we've simply not done enough.
"It's the same group of players that have put in some outstanding performances this season and yet here for long periods the performance was unacceptable."