An ubiquitous theme in nearly all of Shakespeare's works is the way in which appearances are often contrasted with reality. The function of clothing and references to it in this brilliant play only serves to heighten the importance of this theme. Consider, first of all, the way in which the Duke disguises himself as a Friar, donning the clothing of a Friar, so that he can return to his lands and spy on Angelo and how he is using and abusing his power in his absence. This of course is a great example of appearance vs. reality as the appearance of the Duke belies the actual reality of his identity. However, we should also note the way in which clothing functions as something of a motif throughout the play as a whole as it is used to refer to this contrast and tension between appearance and reality.
Consider for example what Isabella says to Angelo in Act II scene 2 about how the trappings of power and the clothing that is associated with it is not important compared to the virtue that every ruler should possess:
No ceremony that to great ones longs,
Not the king’s crown, nor the deputed sword,
The marshal’s truncheon, nor the judge’s robe,
Become them with one half so good a grace
As mercy does
Angelo himself recognises the truth of these words in his soliloquy in Act II scene 4, when he openly admits that people are often impressed by the appearance of power rather than its reality:
O place, O form,
How often dost thou with thy case, thy habit,
Wrench awe from fools, and tie the wiser souls
To thy false seeming!
What is significant about this soliloquy is that Angelo identifies how his apparent moral gravitas is actually just a pretence, and is part of the appearance of his power that his clothing gives testament to.
Therefore we can see that throughout the play clothing is a powerful motif that points towards the central theme of appearance vs. reality, especially when related to positions of power and how that power is used and abused.