Well, how much have we actually seen of her being smart vs an idiot?
She won her first invasion with strategic use of shape-shifting and successfully wiped out Timbucktoo.
After that we don't know everything she was up to, but she eventually invaded Trot with a trojan horse after shape-shifting didn't work.
She lost because of Celestia who was not only stronger than her, but also unknown to her.
With no time to prepare a plan, this fight was lost due to the element of surprise, not her stupidity.
She managed to trick the dragon that was going to eat them and escape into the wild for hundreds of years. Smooth talking + a bit of mind control.
Then she managed to sneak into Canterlot and kidnap Cadance undetected. Her act fooled everyone but Twilight, including Celestia. That doesn't sound dumb.
She did dismiss Cadance's efforts to restore Shining Armor with love as an "absolutely ridiculous sentiment", which is odd considering her own species does this on a regular basis. You could chalk it up to stupidity, but I prefer to think it's ignorance of how unicorn magic works. Especially considering that she asks Twilight to join her and give her magic lessons in issue 4.
Her plan in the comics worked smoothly until Twilight proved to be better at channeling the magic surge than her. Again, she either was too stupid to see this coming or was just unaware of how it would affect Twilight since she didn't study magic.
And in FIENDship, she tricks Twilight (the one pony who wasn't fooled by her Candance act) into letting her go. Her drone put on a better act than she did as Fake Cadance, but that is to be expected since she only had a short time to learn about Cadance, while the drone probably knows the Queen super well. Gotta give her props for her ruse.