Well, I think it depends on how define “bad” but when it comes to useless? I’d guess only 10% to 15% that just don’t have any use. These need buffs.
I’d guess another 20% to 30% have very narrow use cases, as in only one type of build can use it. Casual players might see these as “useless” but only because they haven’t played any builds that might find a use in it. These items are actually okay and suffer not so much because the item is bad but because build diversity is so poor. Buff the related build, and the item will become desirable.
I’d then guess another 30% or so are better than nothing, but not very competitive with mid-level rune words. So you only use them until you have the runes and base you need to make the rune word. These items actually do need some help I think, more something that differentiates them rather than a straight power boost.
That would then give us maybe 20% that while not best in slot, are really pretty good, and better than is often appreciated. These are fine.
And another 5% to 10% that are in fact best in slot. These are also fine, but ought to be looked at to consider if some additional options are needed. For example, caster gloves. There really ought to be other choices with Fast Cast besides Trang’s and Magefist. Why don’t Sanders Gloves have faster cast instead of Increased Attack Speed?