The Baggies failed to protect their 3-2 lead as Cardiff scored an equaliser through Mats Moller Daehli with seconds remaining.
Mel has insisted that he is encouraged by his side's performance.
"I believe we did well on Saturday," he told the Express & Star.
"Leading the game 3-2, we should have killed the game off. We have really sinned. We sinned as an inexperienced team. It's normal that people won't be happy. There is a lot at stake. But we trust in the players.
"What I'm really concerned about is that we played a good game. We played very well during the first half and the players put in a good shift so I can't criticise them for that."
West Brom travel to relegation rivals Norwich City on Saturday.