Not to start a debate or anything, but what would you have to say if an OC was an alicorn because they're from another universe (Most possibly non-pony related) where they are a divine being? This is the ONLY reason some of my OCs are alicorns and they are far from mary-sues/gary-stews because they have their own problems to deal with which are usually much bigger than themselves. Heck, my OC almost died in battle against a greater force.
I think there's a difference between having a personal OC, and a character from a fanfic. It's hard to mix the two. I say, alicorn OCs aren't a problem in fanfics as long as they have a role and make sense, but putting them in place as YOU as a person..doesn't fly unless you have a really REALLY good reason or done something amazing in life.