Nah you can measure it. For example we can describe everything that makes up Kuma's character without throwing in what we personally like or dislike, and based on how much we know of the character, we can recognize how much depth and complexity said character has because at the end of the day these characters are created with a purpose from the writer. No matter if we like it or not, thus far Kuma's a good guy thar holds loyalty to those that he holds dear, and had a shitty life. Sure there are details I'm leaving out, but this pretty much describes his character, which isn't much in comparison to other characters within One Piece. This doesn't take my subjective likings into consideration, but simply taking what is given by the writer, and analysising what it is. If you consider someone like Kuma to be a good character, then fine, but it doesn't change the fact that Kuma isn't doesn't have much complexity nor depth in his character.