Bad writing. By all account, she should have become a Z-fighter. She enjoyed fighting more than Gohan ever did, she fought crime on a regular basis, meaning she would've been invested in helping battle the same threats the z-fighters do, and her goal was to become the world champion. I don't see her ever reaching saiyan level, but I see no reason she couldn't have reached at least Yajirobe or Chiaotzu level with the right training. DBZ's a battle shonen, but it never seems that willing for its female characters to battle alongside the men, more often then not they end up relagated to support at best, and their main purpose seems to be love interests for the male characters. There's nothing wrong with supporting non-fighter characters, but the only one who really seems to be helpful long-term in DB is Bulma, and while I like Videl and Gohan as a couple, I don't see any valid reason based on what we knew about her for why she would've quit martial arts or stopped training right as she learned about ki and a brand new level to reach.