When I see this question I can only be sad for people who can consider it perplexing. Yeah - if you have a sufficiently boring muffin and a sufficiently dense cake, you could see them as pretty similar but … eat better food.

Put some good stuff in your muffins (blueberries, poppy seeds, nuts, chocolate, etc) and learn to keep your cakes lighter and fluffier (especially for cupcakes) instead of rich like a muffin.

As others have said, they are different batters, the cooked result of both should be pretty noticeably different with it without frosting.

That article makes it clear it is less harmful.