i think most of the "bad" alicorn OC's come from new members to the fandom trying to make their OC more likable, when many people start in a fandom they try to make their OC interesting and powerful, but in most cases people over power their alicorn, unicorn, Pegasus, or earth pony OC. people just need some time to develop their OC's and their powers and personality, i think it's best to come up with backstory/personality first then what the Oc look's like. hopefully this will help new members of the fandom make a good OC.