You are right to recognise the emphasis that is placed on clothing in this brilliant and often ignored play by Shakespeare. What is notable as we read through the pages of this play or see it on stage is the way in which the relationship between the clothes that a character wears and that character's identity is such a massive recurring theme. Just to take one example, consider the way in which Cloten is offended by Imogen's statement that the "mean'st garment" of Posthumus is actually worth more than Cloten himself is. We can see therefore that in this play the kind of clothing that characters wear is automatically associated with their worth and value.
Of course, the emphasis on clothing draws attention to the key theme of appearances vs. reality. The insult of Imogen actually draws attention to the way in which Cloten is disguised as a prince, but actually fails to act as a prince should. The importance he places on his clothes and his princely trappings is belied by his un-princely manner. This further explains Cloten's anger that Imogen should favour the appearance of somebody who, in the wordly sense, is worth "less" than a prince. What is notable is the way in which the play presents appearances as being profoundly problematic, and it does this by implicitly contrasting Cloten as a prince who is actually very unprincely in his behaviour with characters such as Arvigarus and Guiderius, who manage to show themselves to act in princely ways in spite of their humble appearance and simple clothing. The more simple and rustic the clothing, the more true the character. This appears to be the general theme of clothing in this play.