Mauro Icardi scored his first two goals since February for PSG at the Stade Auguste-Delaune as the Ligue 1 champion made it three victories in succession without conceding.
PSG started with Kylian Mbappe and Neymar together for the first time this season, while Angel Di Maria and Icardi were also named in an attacking line-up.
Despite the comfortable nature of the victory, Tuchel was not overly pleased with his side's performance against a Reims side without a win this term.
"I liked it in the first half, but I don't understand why we missed a lot of chances," he told Telefoot. "After 20 or 30 minutes we should be three or four up.
"But I am not happy with the second half that we did. It wasn't serious. There was not the same aggressiveness and the same intensity
"The plan was to put Neymar close to Mbappe. We chose an offensive structure. We tried something and I have a lot of confidence in my team.
"We had a lot of chances and had 70 per cent of the ball. If we put three or four chances away after 30 minutes, we have a different analysis.
"So we can see that we missed a lot. The efficiency was not there. You can't play a game like that. In the second half, we weren't serious."
Match-winner Icardi struggled towards the end of last season and did not feature in PSG's Champions League semi-final win over RB Leipzig nor the defeat to Bayern Munich in the final.
And he is relieved to have got off the mark in 2020-21 against Reims, the ex-Inter forward scoring from two of his three attempts on target and creating a joint-high four chances.
"It was important for me to score these goals," he said. I'm happy to score. "I was relaxed during my non-scoring period as I was helping the team defensively. Now we must continue.
"I'm happy to create chances for my team-mates. We could have put in more to kill the game faster, but satisfaction prevails anyway."
Asked if PSG need another forward before the transfer window closes, Icardi said: "This is a question for the directors. The players are here to play."