There are quite a few enemies that should have their required banner kills lowered. I'd argue that even several enemies with reduced kill requirements should be reduced even further, like Doctor Bones or the bride and groom. Honestly, those enemies are so rare that they should need only three kills. It also wouldn't be a balance issue because by the time you've managed to kill three of them, you're going to be geared enough that they won't be a problem. Even among the enemies that aren't reskinned zombies, there are a number that could stand the same treatment. I don't remember them all, but when I was grinding out all banners in 1.4.4 I noticed that there were a few whose banners you'd never get without specifically grinding for them. Many of them are rare enough that even if you were able to benefit from a banner before you outgear the enemies, you realistically won't get any use out of the banner because who would want to carry around an entire inventory of banners on the off chance that you run into one?
Basically, I'd summarize it as "if the banner requires the player to deviate significantly from what can be considered regular play, the kill requirement should be lowered". Otherwise the item really only exists for the sake of existing.
