Personally, if I saw someone published a ton of papers during their PhD, I'd question the quality and importance of their research. Quality tends to decrease with quantity in my experience. And it appears too easy to fake importance. In my field I watch out for vague words like "novel" and "fundamental" in paper titles as evidence that the work is probably overhyped. I'm fairly well read in my field and I can sense the disappointment in people when I tell them that their "great" idea was done before, particularly when it was done better and/or a long time ago. (I take no pleasure in doing this. I just wish people spent less time reinventing the wheel.)

I think I could have had 10 papers during my PhD if I focused on easily-published things and "salami-sliced" my papers. But I focused on what I thought was more important and made the scope of each paper larger than normal because it saves readers time.

Proxies that are characteristics of the researcher might be better (or at least should be considered), like the willingness of the researcher to put "skin in the game" so-to-speak. I'd have a lot more confidence in research conducted part-time by someone on their own money than someone using another person's money (which has much less risk). Taleb has written about this as I recall.