The 37-year-old player-coach netted the winner after 17-year-old substitute Joe Ralls had cancelled out Matty Fryatt's opener with his first senior goal.
Ralls was an early replacement for Kenny Miller, who hobbled off with a groin problem that could see him ruled out of Scotland's Euro 2012 qualifier with Liechtenstein next Saturday.
To compound a tough day for Cardiff, referee Colin Webster reversed a decision to award a penalty to the visitors just moments before Barmby's winner.
"Nick has always had a very positive impact for us," Pearson said. "He is a player who brings quality to the team.
"We're not just a squad of young players but our young players have great potential, and Nick is a role model for them.
"During the week he's more of a player than a coach, but this is his club and it means a lot to him.
"The most important thing for me is that I know he can still perform. He's not just a cosmetic extra."
Hull, who ended Cardiff's six-match unbeaten run, have now avoided defeat in their last six matches themselves.
"There are lots of teams that will think they have a chance, but we've made some great progress," Pearson said.
"The players can be very pleased with their performance and there is an air of confidence about them."
Cardiff felt aggrieved after referee Webster performed a U-turn on a decision to award a penalty to the visitors after Jack Hobbs' trip on Filip Kiss.
The referee, after consulting his assistant, signalled for offside, and Barmby notched the winner moments later.
"He was clearly offside so it wasn't a fractional thing," Pearson added.
"Common-sense prevailed and it would have been unjust to have conceded at that point."
However, Malky Mackay was dismayed by what he called a "game-changing decision".
The Cardiff boss said: "The linesman has apologised for not putting his flag up, but that makes no difference to me. The play went on into a second phase and if it was offside it was marginal."
The penalty controversy marked a bad day for Cardiff, who had to reshuffle following Miller's early injury.
"He suffered a groin problem in the first minute and we had to restructure and put square pegs in round holes," Mackay added.
"But I thought we were good enough value to get something out of the game."
David Marshall tipped over Robbie Brady's curling effort from 25 yards - the first of several fine saves by the Cardiff goalkeeper - as Hull carved out the better openings in the first half.
Six minutes before the break, Martin Pusic found Fryatt, who scored with a low right-footed shot from eight yards.
Martyn Waghorn and Robert Koren both went close from distance and Marshall smothered Fryatt's effort as Hull dominated.
The visitors stunned the hosts though with an equaliser against the run of play after 62 minutes. Ralls, who had signed his first professional contract earlier in the day, produced a brilliant lob with the outside of his left foot to beat Adriano Basso from 25 yards.
However, with Cardiff still smarting from the penalty controversy, Barmby latched on to Brady's low cross from the right to score the winner with 19 minutes remaining.