Most of us wear disguises. We figure out ways to make ourselves seem smarter. More attractive. More wealthy. More intelligent. Just better somehow.
Most of the characters in Pride and Prejudice wear disguises, and much of the irony in Jane Austen’s writing is shown when a character’s words are in direct contrast to their actions and disguises.
Lady Catherine disguises herself as a woman of accomplishments– she says she the best natural taste in music and owns the finest instruments, yet doesn’t even know how to play. Mr. Collins disguises himself as a scholarly and charitable man of the cloth, but is more focused on collecting tithes and casting judgements. Caroline Bingley looks down her nose at the Bennet’s poor connections, yet she herself is the daughter of a tradesman.
Mr. Darcy disguises himself very little, mostly because he has no need of disguises himself. He already has extreme wealth and superior intelligence. He doesn’t crave attention or a wide circle of friends and acquaintances.
Simply put, as Colonel Fitwilliam comments, he will not give himself the trouble.