These sound very much like something you would get from a chain coffee shop, but they have the benefit of being homemade, so they aren't dense and claggy like most shop bought muffins. The recipe I based this on was Nigella's for granola muffins. I initially thought berries would be a good addition, and then adding something yogurty seemed like a natural progression- one of my favourite breakfasts in muffin form.
The yogurt frosting is simply yogurt and sieved icing sugar, mixed together. But I was really pleased with it, as it has a slight tang from the yogurt and it sits softly on top of the muffins. It does though up a dilemma, however- if you want to eat the muffins warm (the best way) you don't want to ice them and let them sit, as the frosting will just run off the muffins. So you can have them cool with frosting, or warm without. There is the possibility of spooning the frosting on a muffin as you eat it, then you get the combination of warm muffin and cooling yogurt.
The recipe for the granola muffins can be found here. I added about 150g berries stirred in with the granola, a mix of blueberries and redcurrants, but I most berries could be used. For the icing, I mixed about 100g Greek yogurt with 4 heaped tsp icing sugar. This didn't make enough for all the muffins, I only wanted to make a small amount as it was an experiment. So if you want to make enough for all of them, I would triple the quantity