Well done, there is nothing grammatically wrong with this sentence. Without seeing the picture it is hard to make any comment as to how logical it is. However I would suggest in this case to us "as" so "I didn't smile as the shirt did the job for me" both sentences are correct but as is easier to say and feels more natural. In English we would say a sentence sounds 'clunky' if it is grammatically and logically correct but there is a more natural way to say it.

I like it. I could easily believe it was written by a US native speaker. I think it's perfectly correct English, but the word and phrase choice expresses informality. I assume the shirt has a smiley-face emoji on it, or a picture of someone or something smiling.

If your shirt has something that would make someone smile or know you are happy just because you are wearing it....you maybe could use this caption.