Adding a bit to Brian Towers' answer, which I think is the best one.
In top-level chess, White has a noticeable advantage from the opening position. Therefore, "win with white, draw with black" is a common maxim: you don't have to aim to win your Black games, but you should aim to win the White games. In other words, you should come out of the opening with some advantage.
Carlsen is being criticized because in his White games, he has very often failed to get an opening advantage. This might be explainable if Caruana learned a completely new opening repertoire (Boris Gelfand did this in his match vs. Anand); however Caruana has simply continued to play the openings he usually plays. I'll highlight what happened after a screenshot of Caruana's laptop was leaked. Some people argued it was a ploy, because the screenshot basically showed that Caruana was looking at the QGD and the Petrov's Defense, which Carlsen ought to already have known. Former world champion Anand said as much, calling the "revelation" that Caruana was looking at the Petrov's Defense "completely useless". But if Carlsen had known Caruana was going to play these openings, he ought to have prepared some kind of line that would lead to an opening advantage. So why didn't he manage? For example, game 7 was a QGD that Carlsen already knew that Caruana was likely to play (if he didn't know it beforehand, he definitely knew it after game 2), and yet he still came out of the opening with no advantage.
A fair question is why didn't Caruana's opening preparation come under criticism. First, it's harder to prepare against Carlsen than to prepare against Caruana: Carlsen plays more openings. When you don't know what opening the opponent is going to play, you have more ground to cover, making things hard. After game 1 when Caruana learned that Carlsen was going to play the Sicilian, he did make some progress in game 3, gaining a small opening advantage. He did it again in game 5, where he reached a complicated position from which he could hope that Carlsen would fail to navigate correctly; and then again in game 8, when his position was even winning.
You'll have to come to your own conclusions about whether or not the criticism is fair. I'm simply explaining why it's happening.