No. If spaghetti sauce had just been spilled on the dress, I would say: It has been stained.

Tainted is more for things with volume... eg water, drinks, food, air... the 'taint' goes through the whole thing. Figuratively, if you say someone's personality is 'tainted', you mean the sour/disgusting/unpleasant feeling is subtle, but definitely there in every part of the personality. Since we look at dresses as a surface area, you'd say something more like "stained". Any of the suggestions above are good, though I'd save "tarnished" for metal surfaces. Just my thoughts! :D

Hi... could be a special way to say your dress is dirty... but it is strange...

No, it's quite awkward. If you're trying to say dirty, it's better to use: Stained, soiled, or tarnished. You can say: Oh my God! Look at your dress, it's been soiled! etc.