Mary Shelley offers small clues to Victor Frankenstein's appearance throughout her novel, Frankenstein. The first description is provided in Letter IV.
He was not, as the other traveller seemed to be, a savage inhabitant of some undiscovered island, but an European.
Based upon this limited description, readers can assume that Victor is white, with defined features and brown or blonde hair. He is most certainly not physically unrefined, given Walton fails to define him as a savage.
That said, later in the letter, Walton goes further in physically defining Victor. Victor is emaciated from his search for the Creature. This means that Victor is very thin and, one can assume, his facial features are even more refined given his gauntness. Walton describes Victor's eyes as being both mad and wild. At the same time, his outward disposition speaks of kindness.
Later, throughout the novel, Victor is constantly described as being ill. When working on reanimating life, Victor puts all else on hold. His own health is ignored. Therefore, one can assume that Victor has both a look of one who is educated yet thin from obsessive work.
Like the Creature, Victor's exact physical description is masked. While the missing physical descriptions of the Creature are apparent (Shelley does not want to paint a specific picture of the Creature; instead, she wants readers to create their own mental picture of him), leaving Victor's physical appearance open to interpretation makes him far more curious to us.
One could assume that the only important thing about Victor's physical appearance is that he is thin and uncommonly ill. This sets up the image of the "pale student of unhallowed arts" Shelly pictured in her dream which was the beginning of the novel.
Victor is young and we may assume good-looking. There is no indication, at least, that he is not hansom. People search for his company and he is respected for his intellect. He is generally liked but at the end when he is described there is a different view of him because of his suffering:
I never saw a more interesting creature: his eyes have generally an expression of wildness, and even madness; but there are moments when, if any one performs an act of kindness towards him, or does him the most trifling service, his whole countenance is lighted up, as it were, with a beam of benevolence and sweetness that I never saw equalled. But he is generally melancholy and despairing, and sometimes he gnashes his teeth, as if impatient of the weight of woes that oppresses him.
Some critics compare Victor Frankenstein with Mary's narcissistic and womanizing husband as well as family friend Byron. So, that could be an indication of his attractiveness.