Did I damage my frame by removing a sticker? I’ve just bought a new bike with a matt finish, and peeled off a sticker from the top of the frame - but this revealed a shiny finish underneath. 

Is this a problem, and do I need to do anything about it?

I’m wondering if the frame is now vulnerable to weather etc. on this spot, or if there’s a risk the coating could start to peel away from the edges.
The shine is likely the residue from the sticker adhesive. I would apply some rubbing alcohol on a soft cloth and see if it comes off. As you haven't damaged the actual finish, it will be fine. I had a similar finish on a Diamondback Response. After its' first wash I waxed it. I thought I had ruined the matte finish. When the wax wore off the original finish returned.