Sarah died. That was devastating. She freaked when she saw her messy mouthed brother after his first bite and caught a bug as a result. If Barnabas had been eating a sloppy joe and got the stuff all over his mouth, took a walk and Sarah saw him, I don't think she would have had the same reaction. The story was for her that Barnabas went away and that was that. A story invented because he was, in fact, dead. Why was he dead? Okay, why was he messy? I guess the first bite was a bit of a trauma for him, along with the whole blood-letting thing. He didn't clean up. The whole thing can be see as a tragic chain of events.
Actually, it wasn't Barn's first bite. The first bite occurred off-camera in Ep #411 - and the only evidence of it that we have is Barn's bloodied mouth/chin/clothes when he suddenly appeared in front of Ben in the cemetery and announced he'd discovered he has remarkable powers and supposedly needs blood to survive (I say supposedly because, well, he survived for over 170 years chained in his coffin without blood until Willie let him out, so within the DS universe one could argue that rather than a real life or death survival need, DS vamps crave blood like an alcoholic craves a drink or a drug addict craves his/her drug of choice, and indeed DS as well as Frid definitely played up the addiction angle to great effect - but that's a completely different discussion
). The night Sarah catches Barn in Ep #414 was at the very least after his second attack - the attackee having been Ruby Tate - and his mouth/chin/clothes were still just as bloodied. Was he still becoming as traumatized as he might have been initially? Perhaps. Or perhaps he was simply still too obsessed with gorging on his food(drink/drug) of choice to care about his appearance.
Though in reality within the storyline we know that Barn was still so bloodied simply so that Sarah would become terrified by him and run away into the night. It was purely a necessary plot convention...