You've only been working for 30 minutes, so why are your clothes (already) so dirty?
You've only been working for 30 minutes, so how come your clothes are (already) so dirty?
You've only been working for 30 minutes, so how did your clothes get so dirty?
「まだ30分しか働いていないのに」は、you've only been working for 30 minutes です。
「そんなに服を汚して」は、いくつか言い方がありますので、見てみましょう。
英訳1 You've only been working for 30 minutes, so why are your clothes (already) so dirty?
dirtyを使うと、伝わりやすいですね。
英訳2 You've only been working for 30 minutes, so how come your clothes are (already) so dirty?
why の代わりにhow comeを使ってもいいですね。
英訳3 You've only been working for 30 minutes, so how did your clothes get so dirty?
get を使うと、汚くなったのがより生き生きと伝わりますね。
「30 minutes」の代わりに「half an hour」を使うことも可能です。
ご参考になりましたら幸いです。