Goals from Jack Colback and Fabio Borini helped the home side to a comfortable victory, securing their Premier League safety and putting Mel's side still within reach of 18th-placed Norwich, albeit it is unlikely they will be caught.
There will need to be a 17-goal swing on the final day for the Baggies to go down, but, regardless, Mel was left with a bitter taste in his mouth following a poor defeat.
Speaking to Sky Sports, Mel said: "I'm very disappointed, particularly with the first half.
"Sunderland defeated us in terms of intensity, in terms of winning the second balls and going into tackles strongly.
"I believe that we weren't as convincing.
"It was a case of putting out fresh players, players who hadn't played as much, but the result wasn't much of a good one."
West Brom go into the final game of the season at home to Stoke knowing that their Premier League status is all but assured due to Norwich's poor goal difference.
Mel, however, wants his side to repay the fans' faith on Sunday and end the season on a high.
"I hope that on Sunday we can change the image that we give of ourselves," he added. "I'm sure the fans won't be very happy."