Mistake in a sense that Gege didnt balance it.
If good side loses something they gain something in return. But it didnt happen.
It ties back to losing streak of main cast.
Like I said, the Ws they get are after massive Ls.
In case of Mahito they lost Mechamaru, Nanami, Nobara, and many human beings.
So, in that sense losing 2 member of the main cast was a mistake. What makes it worse is that Yuji didnt get a massive power up. It does not have to be an asspull one. Just that culling games arc was bad and did not help main cast to level up.
Keep in mind that we were mainly talking about the increase in criticism of JJK, which has gradually increased. So, I argued that it is not only because of chapter 236 or Sukuna. Gege has been making unusual decisions, which more or less make JJK unique, but it has its own issues.
For example, the readers who have read other shonens, they expect MC to shine, they expect main cast to do better. But this is not the case for JJK.
This makes JJK good because it takes bold decisions, but understand why people also criticizes the story because the pay offs does not feel good at all.