When did yahoo answers become a dumping ground for idiotic questions?

Because people can ?

From the moment they decided they were too cheap to moderate the site with professionals and instead turned it into a "community" policed site.

It's a dumping ground for idiotic answers, too.

When Yahoo laid off most of the staff, before being sold to Verizon, several years ago. Without a YA Team to oversee the site and handle serious violations via direct contact, the trolls and abusers started to become more bold, and multiply. That whole 'when the cat's away" scenario. Verizon only briefly seemed to have interest in YA, when they had an Oath-YA Team for a few months last year (late Spring - Oct). Alas, that team was disbanded, so the abuse of the site has become significantly worse since the only recourse responsible users have is to report posts and HOPE enough other users will do so as well (even then, it has minimal impact).

It was probably a slow gradual transition.

There have been idiotic questions on the site since the beginning.

If you mean the yahoo!products-->answers CATEGORY....some level-7 people are moving clear violations to that category so you can report them.

troll heaven for many years

At the same time when Yahoo started deleting questions (and answers) willy-nilly, I think they started padding with idiotic questions.

They apparently wanted to control the content as well as the questions.

One person showed how these questions usually are from robot accounts with no history of their own.

since site went to hell

I can't remember when it wasn't.