Transparency is hard and VRChat cannot control all conditions a user can place a sticker down in.
Most stickers do not need to be transparent; whilst the funniest and best stickers _are_, as users have quickly discovered that the best way to use a sticker is to slap a modifier on someone else's, most stickers are going to be one or more opaque forms on a transparent background.
As such, VRChat could allow for a user to place either a transparent or non-transparent (cutout?) version of a sticker they've created down. This would allow for a good level of flexibility. This might also be a vector for performance, with distant stickers that are not occluded being swapped to cutout to help lower pixel overdraw.