Interesting -- I suppose that if you don't mix the ingredients effectively, the result is that you get concentrated pockets of raw product which correspondingly taste much stronger :-(

You definitely don't want to be able to taste the baking powder, which is why scones (which require a *lot* of raising agent proportionate to the flour) are better made with bicarbonate of soda rather than baking powder -- it's more powerful and you need less of it.

American muffin recipes tend to emphasise that the mixture ought to have lumps of unmixed flour in it; I'm not sure why, as cupcakes in this country are normally mixed 'to a dropping consistency', i.e. until the mixture will fall off the spoon, and are definitely not supposed to have dry lumps in them!