Mig welding is easy once you know what your looking at.
I found it easier to learn on thicker pieces of steel as it wouldnt blow through if it got too hot. As said, penetration is the key, especially if your going to grind it off.
My tafe teacher always said to us as a 1st year apprentice.."if you think it looks good, then its probably not hot enough or your going to quick" On thin sheet metal a mig weld rarely looks nice.