I’m assuming you are talking abut federal judges appointed under Article 3 of the US const.
All district court and Courts of Appeals judges are able to appoint clerks who do research and draft opinions for the judge. I’m not sure how many. ISTR district court magistrate judges (one level lower) were able to name 2 law clerks. So hiring competent clerks will hide a ton of deficiencies on the judge’s behalf. A judge can essentially tell their clerk, “I want to decide X. Draft a decision that supports that.” Then the judge can just sign it.
The judge can also have the clerks review any written pleadings by the parties, and prepare memos for the judge - “cheat-sheets” if you will.
In addition to the personal clerks, the district and appellate courts have general clerks, who can be assigned research and writing tasks.
So basically, the judge just needs to keep their mouth shut during open court, so as not to appear a fool. The basic response is, “I’ll take it under advisement.” Then go back behind closed doors and have your staff do your work for you.
Federal rulings and decisions can be appealed to the next level, tho it is costly and time consuming. And there is an administrative body (I forget the name/acronym) that reviews judges’ productivity, and how often they get reversed/remanded.
There always have been stupid, unqualified, unreasonable, and biased judges. It is pretty obvious to those who practice before them - and I would assume, to their colleagues. Most federal judges aren’t inherently stupid people. They know they have a good gig, and IME even the worst tend to learn how to conduct themselves so as not to flaunt their incompetence.
There is definitely a learning curve. No matter what a lawyer’s experience, it does not DIRECTLY transfer to being a judge. But even the procedural and substantive material can be learned. And there are quite limited numbers/types of actions that have to be taken IMMEDIATELY. At the very least, a judge can always call a recess - without giving ANY explanation, and go back to chambers and ask his clerks “WTF do I do in THIS instance?” Federal judges are - in many respects - little kings/queens.
Not sure if this is the sort of thing the OP is asking.
I’m not sure the “qualifications” of the recent appointees is nearly as big of a deal as the philosophical bias they bring. Wouldn’t have been as big of a deal had previous administrations been able to fill vacancies.