Reading a person's description I came up to the adverb-adjective construction "ideally bald". I understand that an idea is an abstraction and it cannot be applied to physical features. How can someone be ideally bald?
It still seems odd, but perhaps Nabokov meant either (a) totally or completely bald (that is, without little wisps or strands of hair to mar the baldness) or (b) with such a well-shaped head that he represented some idea of bald beauty.
A commentary on this actual passage says "This propensity for creating combinations of nouns and adjectives or adverbs and adjectives that are simultaneously natural and jarring springs from Nabokov's Russian."
Another person says "Professor Wood said that Nabokov, despite his eloquence, wrote English like a foreigner. His distinctive voice is a foreign voice, with a freakish attention to idioms, colloquialisms, and puns. His prose is “ecstatic” (to quote Updike) because it is slightly odd to our ears, containing cadences and phrases that would not have occurred to our native English-speaking minds. By punching the lights out of our linguistic brain circuits, these phrases almost force you to think laterally."
...next to an empty seat and facing two empty ones, was none other than Professor Timofey Pnin. Ideally bald, sun-tanned, and clean shaven, he began rather impressively with that great brown dome of his, tortoise-shell glasses (masking an infantile absence of eyebrows), apish upper lip, thick neck, and strong-man torso in a tightish tweed coat, but ended, somewhat disappointingly, in a pair of spindly legs (now flanneled and crossed) and frail -looking, almost feminine feet.