Most of the alicorns OC's are bad. Either because they are way to overpowerd or because the creator made them ridiculously underpowerd. The secret is to make the OC just powerful enough that it's believable but not powerful enough that they are equal to (or greater than) the rest of the alicorns combined. Another thing that annoys me about alicorns OC's that sometimes people think that making an alicorn who can't fly or use magic is a good idea, the whole point of alicorns is that can both fly and use magic. If you're going to make an alicorn who lacks one of these abilities, why not just make a pegasus or a unicorn?