Azz, you are playing with fixed phrases there. Your sentences c, d and e lose that.
Something (or someone) is in good shape or they are in bad shape. No article required.
To refer to your original post, I think it would be better to avoid the use of the article by separating the two thoughts:
a. She is in good shape; too good to play the role of an overweight lady.
b. He is in good shape; too good for me to beat him in a race.