Onana is not a bad goalkeeper, he is a goalkeeper who has good abilities and one of the reasons he is at Manchester United is also because he has the ability.
But the problem is since he was brought in, he always makes mistakes that ultimately make the team get unsatisfactory results. He repeatedly does that, and that is what ultimately makes many people comment negatively on him.
Onana was a great goalkeeper when he was at his previous club, Inter Milan. In fact, he managed to keep Inter Milan's goal rarely conceding at that time. So, Man United is also interested in bringing him in. But when Onana came to Man United we often saw Onana make mistakes. Several blunders even resulted in Man United conceding a goal. I don't know what happened. But what is clear is that Onana doesn't seem to be mentally prepared for this club. But I'm starting to think that the real fault lies in Man United's back line, which is too easy for the opponent to pass. So the goalkeeper has to work harder and of course the goalkeeper ends up being overwhelmed.
It is very easy to notice the goalkeeper's blunders when the defense line is weak. Manchester United's defense line has been weak for long; Onana will be the one covering it up and also make terrible mistakes that will be more than the saves he has made before, which is very bad.
What's the essence of having beauty-savings in matches, but the one that the team needed the most for a match win, you will leave everyone disappointed with your performance.
What is clear is that a club will still fail to win even if they have a great goalkeeper if their defense is very weak. Because no matter how good a goalkeeper is, if he continues to receive continuous attacks, he will eventually make mistakes too. So it requires the solidarity of all players to be able to cover each other's shortcomings between one player and another. And we know that at MU, blunders are not only made by the goalkeeper, but quite a lot of defenders and midfield players do it too. I remember that during De Gea's time we commented a lot about the blunders made by the team's defensive players which made De Gea considered to have decreased in quality. 
The truth is that Onana was one of the best goalkeepers in Italy before and had an amazing run to the Champions League final, as well, but when you are in a country that isn't as competitive as the Premier League and those Inter defenders were good that he didn't have all that much to do in goals so he wasn't exposed for what he was.
The leagues he's been in, combined with the defenses in front of him, have made him look better than he actually is. It's just a shame it took a move to Manchester United to show just how bad of a goalkeeper he is which has proven bad business for United but also United haven't had a settled back 3 or 5 which ever way you look at it which means they GK is tested more and has to do more.
Onana is clearly not the only problem at Man United, but one of the problems. IMO