Mallinder's side stumbled through an unconvincing second-half and he cited his team's line-out and goal-kicking as the most disappointing aspects of their performance.
"You want to get on the front foot but our lineout didn't function," said Mallinder.
"We were unfortunate to lose lock Christian Day very late in the day through food poisoning and this affected our lineout calls.
"The goal-kicking was frustrating, both had off-days but last week Bruce Reihana kicked four from five and the week before that Shane Geraghty kicked four from five.
"We were very nervous at the end. Castres played well, they got stuck into us and made life difficult.
"Allowing them a bonus point was not ideal but I'm confident we are still a good side and can go to Edinburgh next week and get a result.
"We won ugly but I'm confident we will do well in this competition and qualify for the next round."