He won the championship in both lower categories at his first attempt. No-one has scored a point in a Williams for quite some time. The team is grossly underfunded and the car is far slower than all the other cars on the grid.
He has outqualified his team mate at every single attempt while driving for them.
If you followed the sport reasonably closely you would know that everyone who is anyone rates him highly.
Thats why he has been signed as part of Mercedes junior team, and was seriously considered to replace Bottas this year.
He regularly beat Norris, LeClerc, Sainz etc. in pre F1 days.
He had probelms remembering which button to push during qualifying, as they are in different places on the Williams, and pressed the wrong one a couple of times, but still got within a few thousands of Bottas.
He is far from average, even by F1 standards..
As for Vettel, yes he did won four titles so cant be said to be useless. But he did have a huge car advantage , a team built completely around him, and his team mates werent usually allowed to beat him.
He has also been allowed to get away with very dubious driving tactics many times which others would have been punished for. It must be a German thing.
He hasnt looked too impressive since he started driving a car that wasnt designed by Adrian Newey.