Whoever invented muffins was really onto something. They're basically cupcakes without the frosting - of course they were going to be popular if you told people they could eat them for breakfast! Not only are they typically baked with sugar and refined white flour, packaged muffins also come with a full dose of preservatives. Muffins are high in calories and the sugar content can cause them to be addictive. When you combine it with a morning coffee there's no wonder you get that mid-morning come down.
Sugary cakes are not a good dietary choice in general, but especially not for breakfast. Giving your body such a sugar spike so early in the day is only going to set the tone for more unhealthy eating. Once your mid-morning come down hits, you'll start searching for even more sugar and caffeine to pep you back up again. It's a vicious cycle!