But here's an article from 2016 about what LendUp was trying to do back then: https://techcrunch.com/2016/01/22/the-loan-dolphin/#.4sfbucn...
I don't know much about LendUp at all, but I do remember the surprise - and feeling it myself - when it was announced that YC was funding a payday lending company. Then I remember the thinking behind it being explained (perhaps on HN, perhaps elsewhere) and seeing that the founders seemed to be making a sincere effort to offer something that was at least just less-terrible than the existing payday lending options around (many of which involve dealing with crime gangs), which made me think "OK, I guess it's worth a try". I presume YC thought the same thing when they invested almost ten years ago.
If it turns out that there's no possible way of making a sustainable business in this space that's less-terrible than that was there before, I still don't think it's reprehensible that YC thought it was worth backing founders that seemed to be making a sincere effort to try.