I usually play my Switch in undocked mode, but I see that people are only doing docked runs. Is there a big difference?

There's no difference, btw all the runs are in the docked category but some runs were made in undocked mode.
I'm going to change that

The difference is that you can't use the touchscreen in docked mode and you would need video proof of a undocked run since you can't get video direct from the handheld.

The game seems to run faster in docked mode, it can help for Race but it's still possible to do the WR in undocked (like what @Félicien2BFélicien2B did).