The reaction picker shows every skin tone of each emoji side by side (screenshot), so the grid is mostly duplicates and it’s hard to find the emoji you want.
Request: show one version of each emoji, with a long-press or hover to pick a tone, and remember my default tone, like iOS and Slack do.
You’re right, the reaction picker in Grok Bot currently lists every skin tone as its own entry, and there isn’t a way to set a default tone yet.
One thing that helps in the meantime: once you react with the tone you like, that exact emoji shows up in the quick reactions row, so the ones you use most stay one click away.