I've never made mint jelly. Made the mint sauce but not a jelly.
Most recipes I've seen for mint jelly include granny smith apples, steamed in the vinegar and the sugar first and then pureed and the mint added last. Other ones I've seen use gelatine but I don't really like using gelatine. I wonder how agar agar would work. It's not made from animals. It would probably leave the jelly nice and clear too, like in the bottles bought at the supermarket.