Pearson was dismayed that his men gave away "soft goals" but insisted that defeat was tempered by the fact that they had set some high standards this season.
Leicester threatened a late comeback when Andy King rose unchallenged to head home from an Anthony Knockaert free kick to make it 2-1 - but Barnes netted his second from the penalty spot 13 minutes from time to clinch the points for Oscar Garcia's side.
The Foxes have now lost three of their last five games and Pearson said: "It is soft goals again. We let them in midweek (against Sheffield Wednesday) and again in this match.
"We've been the architects of our own downfall and it's happened in the last two games.
"We're not firing on all cylinders but the disappointment is tempered with the way we have been setting some high standards this season."
Leicester conceded their third goal when skipper Wes Morgan nudged the back of Barnes, and Pearson complained: "The penalty looked soft to me but these things happen.
"At 2-1, there was a possibility of us getting back into it as we created some chances.
"Well done to Brighton, they are a good side, but we didn't play up to the level we would expect. We didn't do it for the entirety of the game."