Translation:This dress was not clean, I washed it.
If it were clothes, it would be them but with only one dress in English it needs to be it.
The word ruha can be translated either to clothes or dress.
So " This dress wasn't clean, so i washed it." - " Ez a ruha nem volt tiszta, ezért kimostam "
" Those clothes weren't clean, so i washed them " - " Ezek a ruhák nem voltak tiszták, ezért kimostam őket.