The Giallorossi edged the Serie B outfit 3-2 on Wednesday having led 3-0 at half-time and Mazzarri was unimpressed by his team's display.
"We did not read the match properly," he said. "We were not ourselves and we never came to life.
"This game should make us reflect.
"The team fell asleep in the second half and this cannot happen."
The outcome against Trapani allowed Inter to return to winning ways after back-to-back 1-1 league draws against Bologna and Sampdoria.
Inter are fourth but level on 27 points with Fiorentina, who are fifth.
The Nerazzurro will be hoping Rodrigo Palacio can find the back of the net, with the Argentinian forward having failed to score in the last three league games.
Palacio is Inter's top scorer with seven goals.
Rivals Parma warmed up for Sunday's match at Inter Milan by thrashing Serie B side Varese 4-1 in the Coppa Italia on Tuesday.
Antonio Cassano missed that game after undergoing surgery due to a nose fracture sustained during last weekend's 1-1 draw against Bologna.
The former AC Milan striker is set to recover in time for the game at the San Siro.
"There is a very good chance that Antonio will play on Sunday night," Parma coach Roberto Donadoni said.
"We will decide what the best solution is for Antonio to play."
Parma have won just once on the road this season but go into the game inspired by a good run of form.
The Ducale club followed their 1-0 triumph at Napoli with the draw against Bologna last weekend to climb to 10th place.
The visitors will be bolstered by the return of defender Aleandro Rosi and midfielder Walter Gargano from suspension.