:?: When someone you're looking at doesn't look smart (in the well-dressed sense), there are many suitable words - often in the form "<adverb>-dressed": badly-dressed, sloppily-dressed, carelessly-dressed...; but not all the words are like that -"scruffy" and "shabby" come to mind; try a thesaurus.
If their hair is disordered, they are "ill-kempt" (perhaps only Br Eng) or "dishevelled" .
A useful idiom is 'looking as if they've been dragged through a hedge backwards'. Example: "What's wrong? You look as if you've been dragged through a hedge backwards."
But if you're using smart in the "intelligent" sense, start your thesaurus search from "stupid/thick/dense/slow-witted".