I think most of us have seen the videos and the competitions. When put purely in a throwing competition, Dak isn't as accurate as other NFL QB's. During games we have all seen where he is a little behind or over a receiver. It happens to him more than other QB's. He isn't that accurate.
Completion percentage is a factor of two things though: Who you decide to throw to and how accurately you throw it to him.
Let's be honest, just about everyone on this forum could have a near 100% completion percentage throwing to a wide open guy with no pressure on us. Its called "playing catch". Even a yard high or to the side is going to get caught.
Frequently bad QB's are guys with strong accurate arms who consistently throw it at the wrong time to the wrong person. Mitch Trubisky is an example of this.
Dak's strength is usually that he takes the plays that are there. It has frequently pissed people off and triggered nicknames like "Dink and Dak". Regardless, when Dak is on his game he isn't trying to thread the needle into tight coverage and as a result, he consistently has a good completion percentage.
Anyhow, they track this kind of stuff
https://www.pro-football-reference.com/years/2022/passing_advanced.htm
Dak is 30th in the NFL in "on target %". This isn't some made up complaint.