I thought the verb was only reflexive in "mum, how did you cut yourself?"
Would a French child use tu or vous for their mom?
The most common is to use "tu" with all the members of your family. Using "vous" for their parents sounds bourgeois and old-fashioned. However some French still use "vous" for their parents-in-law.
The French doesn't translate the English precisely here! I've reported it, if others can maybe Duo will change the English or the French to match.
'Why is comment as-tu es coupee' wrong?
I'm not French. You're combing as and es which seems very odd. You don't usually use avoir for the past tense of reflexive verbs.
If the cut was done to you by someone else maybe comment as-tu été coupée might work??? But not as-tu es