If this is her clothes, why not suas roupas?
Yes, it seems that both "sua roupa" and "suas roupas" are both correct. See here: https://www.duolingo.com/comment/1025077
Another example of the flexibilty in translating singular/plural nouns:
Eu compre muita roupa nova. (I bought a lot of new clotheS.)
Since "roupa" is a collective noun, I think of it as "clothing" (for which "clothes" is a, perhaps less formal, synonym).
Yes, and as a collective noun, I think that "outfit" is a good translation for 'A roupa'
I just discussed this with a Brazilian friend. We came to the conclusion that "a roupa" means "outfit" and "as roupas" means all of the clothes she has
Can someone explain to me why the "me" comes before "mostrou"?
there's an in-depth explanation on pronoun placement here: https://www.duolingo.com/skill/pt/Clitic-Pronouns
What about ela me mostrou roupa dela
I'm not an expert, but I think it should be "a roupa dela"
Edit: A Brazilian friend of mine said that "a" is necessary because it's referring specifically to what she's wearing or what outfit she has bought at the store.