Can you give me a unicorn character who is worse? I'm being literal here, I can't think of any. Mind you Starlight's presence managed to get me to stop watching the show in Season 6, so maybe they came up with someone even worse later.
As a villain she had some major flaws in:
1. Being inexplicably powerful (still a problem)
2. Having a garbage motivation
3. Being yet another Twilight expy. If you count Trixie (some do, others don't), she was the fourth 'Twilight Unicorn'
Her villain finale was pretty much trash too. The entire first half of it is basically irrelevant, and the second half is Starlight explaining her complete trash fire of a back story.
Sure. But that's all an assumption. Maybe she had the potential to reach a national threat, but she never got close to that stage. And it's just as fair to say that there is nothing saying that she could brainwash more people or promote more to demon status.