Note: The spoiler below includes Dragon Ball Super material and if you are watching the English Dub avoid it.
[spoiler]I won't list Yamcha because he is my favorite character, so its obvious I will speak a lot about him. I will be saving a topic for him maybe weeks or months later.
My list, not on order:
1. Tien: I based it on his treatment in Tournament of Power. I can see that Toriyama prefers to use him over other characters maybe because he has some unexplained fondness of him or because of his unique moveset, but other than that his performance is poor in the Manga because he was less important in it. Also in the show, he was overshadowed by Goku and Vegeta and even Master Roshi (someone who caused great controversy) in all episodeds that were supposed to be about him. Tien's spotlight is easily taken over by other characters and people may blame it on the fact that he is "boring" in personality but this only happened because no one gives him enough spotlight to give him any qualities.
2. Goten: Rumor has it that he was intended to replace Goku. He did not replaced him but he seemed to have an identity of his own in the Majin Buu Saga but he became less than a character in Dragon Ball Super to the point he only appears when Trunks wants to talk with him or to become Gotenks. He has hints of becoming mature but that's about it. He is underused and this causes me to think Akira Toriyama regrets Goten's existence. The fact that Gotenks is more used than Goten alone is insulting as well.
3.Majin Buu: The pink blob was more relevant in his own story arc but after that he is continously avoided by Toriyama and/or Toei. This is more because of his conflict solving abilities of his and less of his personality. Majin Buu always finds a reason to avoid participating in a main fight because he had been sleeping through the whole show ever since he fought Beerus. If Dragon Ball Super continues or a new show is aired, we may not see Majin Buu fight Frieza or a bad guy if he keeps sleeping. It makes me wonder what's the point of keeping him if no one uses him.
4.Chiaotzu: Like Tien, he is underused because he is "empty", Unlike Tien. He is more "blank" both figuretavily and literally. And this is because he is demoted to a background character. Even Goku hardly notices his existence and didn't even considered him for the Tournament of Power when he was close to him. Its like he just disappeared after killing Nappa and Tien and some people are pretending he is still alive and this kind of depressing.
5. Puar: Puar's problem is that s/he is ignored by Dragon Ball Staff because s/he only appears when Yamcha is around and we all know how underused is Yamcha. This is probably based more in indifference than actual hatred to the character.
6.Chichi: Unlike the other characters listed and not listed here, she is the only character I know that is confirmed to be disliked by Toriyama and this is probably because he didn't like her personality and the fact that he claims to not know how to make emotional moments, the only way Chichi could show her sweet side.[/spoiler]








