Most people already want to reduce randomness in the game, especially around crit. This would simply add more, so it won't be particularly popular on that front.
Other than the randomness, the biggest problem I see with this item is that it if it affects non-critical attacks, it affects everyone to varying degrees. Mages would probably suffer the least, but not getting a kill because your auto was glancing will still feel bad for mages. Fighters would suffer more from this item, as several of them use autos for significant portions of their damage (Jax, Irelia, Fiora, even Riven, Darius, Renekton). If it only interacts with crits, this objection goes away, but it wasn't entirely clear from your description.
Also, there are already two items that kind of do what your new item would, but without the randomness,
. Ninja Tabi reduces all auto attack damage by 12%, on top of whatever armor you have. Randuin's has armor, but only gets bonus reduction against crits, while providing a variety of other effects useful for reducing ADC damage. I don't know if there is need for more items with effects like these, but I, personally, would rather see consistent effects than random ones.
I went along with the assumption that ADCs in general need toned down for the sake of discussion, but I'm not sure it's true, let alone to the extent that new items need to be created just to deal with it.