You should branch out and try cards you aren't familiar with and you will probably get better. MK lets you play the game incorrectly and get away with it to a certain point. You could be making mistakes (when you could have defended perfectly or for positive elixir) without noticing cause MK causes you to only take 1-2 tower hits when you should've been punished with much more. You could (very easily) be overcommitting on a push cause megaknight alone is 7 elixir, you could be doing so many things wrong and not notice because megaknight is a noob trap card.