Trailing 2-0 to the Canaries deep into the second half, the 26-year-old fired a successful 86th-minute strike to earn his first goal of the season and give the Black Cats hope of salvaging a point.
"I was pleased to score but the result overall is very disappointing," Richardson told Sunderland's official website.
"We were on a high after the [4-0 win against Stoke], but we didn't play well tonight. We had possession in the first half, but it wasn't effective.
"It's a shame because we have let the fans down and I am sorry for that."
Sunderland, who are currently 14th in the table with five points from six games, will host West Bromwich Albion on Saturday.