If your baking soda/powder isn’t fresh, adding a tsp in a glass of water, it won’t fizz up. That’s a good way to check. And without knowing the recipe for the cupcakes, it’s hard to troubleshoot. Adding a link to the recipe would be helpful.
-- Creativity is God's gift to us. Using our creativity is our gift back to God. Clarky's Cakes 😎
It could be that this particular recipe just does not mound. I would suggest trying a different recipe, if the rounded dome is your desired shape. The reasons the edges of your cupcakes are browned is because your muffin tin is dark. Using a shiny, aluminum tin would distribute heat more evenly and prevent the over-baked edges.
Great tip for testing baking soda/powder, June! I never knew that.
-- Sugar Sugar by SSmiley