This question is a bit tougher than its simplicity would lead somebody to think. The reason for that is because The Hitchkiker's Guide to the Galaxy does not give the reader a lot of details as to what exactly Arthur Dent looks like. The opening chapter gives some clues, but beyond that, the reader has to imagine what the rest of his appearance would look like. The best the text does is the following:
He was about thirty as well, dark haired and never quite at ease with himself. The thing that used to worry him most was the fact that people always used to ask him what he was looking so worried about.
That isn't much detail. He's fairly young at 30, despite what my students tell me. Dark hair. Nothing about how long the hair is or eye color or height or anything. I would imagine Dent has some early aging lines on is face and that is why people assume that he is worried looking most of the time.
The movie version shows Dent in his bathrobe and slippers the entire time. I'm good with that, because the text does say that he put on his slippers that first morning. I can't imagine that he would wear slippers and slacks. The other reason I like the bathrobe and pajamas look is because the text mentions that Dent thinks he must be hung over.
Passing the bathroom he stopped to drink a large glass of water, and another. He began to suspect that he was hung over. Why was he hung over? Had he been drinking the night before? He supposed that he must have been.
If he feels like he is hung over, I guarantee he isn't going to feel like putting on anything other than what he woke up in. I suspect that in addition to the pajamas, bathrobe, slipper, and lined face, Dent has a nice five o'clock shadow beginning. That's just what I picture him looking like since he's waking up groggy and thinking that he is hung over.