Nope. Princess Celestia made her an alicorn because she was ready. If she was a mary sue then she probably would have been an alicorn in friendship is magic part 2.
They can be anything you want them to be! This is your oc. They could be a griffon,
a changeling or even a crystal pony! Make it as unique as you want it to be.
Same easiest pony for me. I've never tried Rarity but I've heard she's really hard.Apple Jacks hat is so hard! Twilight is easiest for me. I've never drawn Rarity or Fluttershy.