Because of her beauty, Makima can do what she was born to do: control. In her role as the Control Devil, Makima is tasked with gaining the complete confidence of humans so that she may exert her influence over them.
To do so, she must adopt a human appearance. Because no one would ever let their guard down around a Devil who appears to be more classically fearsome, which would essentially render Makima useless, she needs to look like someone who can be trusted.
Because Makima must lull her victims into a false sense of security to achieve total control, this is one of the reasons why she is so appealing.
In addition, Makima reflects the fear of being controlled and the dread of feeling dominated, which can extend to the fear of being manipulated, monitored, or coerced into doing something by someone who has authority over them.
The most significant concern humans have is that they will one day be subjugated to the head of another human being, be it a superior, an organization, or even the government. Since this means that fear of control is almost entirely associated with humans.
Since the appearances of Devils revolve around the idea that is feared, it makes perfect sense for the Control Devil to have the form of a human.
Control is also a form of hidden activity. Because it is not as obvious a picture as a pistol or a leech, it is pretty tricky to identify when a person is being influenced; hence the appearance of the Devil must also hint at this.
If Makima’s appearance as the Control Devil was as prominent as that of other Devils, it would not only give her away, preventing her from achieving her aims, but it would also go against the actual definition of what it is to be a Devil.
Because of this, Makima appears human, and her frequently cruel yet deceptive nature fits in nicely with this appearance.