Juan Cala scored the only goal of the game to hand the Bluebirds a vital win in their fight against the drop, leaving the Saints to rue their missed chances at St Mary's Stadium.
Pochettino told BBC Sport: "We are disappointed. If you analyse the game, we didn't deserve to lose but it is a learning curve. In football it's not just enough to play well. You have to be more clinical and aggressive and that is what we lacked.
"We were dominating the game but in final third we were lacking the attacking edge we needed and their keeper was also making some outstanding saves."
Southampton are eighth in the Premier League with 48 points from 34 games.