I dunno. I think for a 70 minute movie, Sunset Shimmer works fine as is. Like I pointed out earlier, she isn't the strongest villain by a long shot, but at the end of the day, she didn't need to be.
Keep in mind that Sunset Shimmer is just an average pony compared to Nightmare Moon, Discord, Chrysalis, and Sombra. And also, being the same age as the Mane 6, she is nowhere near experienced enough to be a "Great Villain" like those others. Like the Mane 6, she is still a young pony/young girl still learning about the world. And also like the Mane 6, she is liable to make some really dumb mistakes when she really should have known better. However, while the Mane 6 have each other, Sunset outright rejects friendship.
That's part of the message Equestria Girls is trying to send (The magic of friendship is everywhere), and IMHO Sunset Shimmer helps execute that message effectively exactly as she was written, someone who destroys friendships in order to gain power. Again, this makes her a pretty weak villain compared to the others, but that's to be expected. She's just a teen/pony who just took things a bit too far (ok, she had sinister intentions, but the same principle applies).