"looking" is definitely a present participle in this context
In the context of "The drow was simply too ordinary looking", the word "looking" is definitely a present participle, not a gerund, assuming we're working in a grammatical tradition that distinguishes between these two categories. Gerunds are "nouny" and present participles are "adjective-y" and "ordinary looking" in this context functions more like an adjective than like a noun: it's describing how the drow looks.
present participles can be "modified" by more than just adverbs
It's an oversimplification to think that a participle can only be "modified by" or associated with adverbs. Participles are (or at least are derived from) verbs, and verbs can be associated with many types of words, not only adverbs.
If the verb the participle is based on can be associated with other types of words, usually the participle can too. For example, "cake" is a noun; the verb "eat" can be followed by "cake" in a sentence like "He eats cake", and the participle "eating" can be associated with the noun (not adverb) "cake" in a sentence like "A child eating cake sat at the table". As BillJ mentions in the comments, there are also "compound adjectives" that can be made by sticking an appropriate word before a participle, e.g. "a cake-eating child". (I won't get into the details of compound adjective formation because I don't know them, and this topic seems tangential to your question.)
in particular, the verb "look" and its participle "looking" can take an adjective as a complement
The verb "look" can function as a "linking verb" that takes an adjective complement. You would say, and correctly so, "The drow looks ordinary" and not "The drow looks ordinarily".
It's equally appropriate to use "ordinary" with the participle "looking". (Although if "ordinary looking" is considered a compound, as BillJ suggests, it might be better to hyphenate it in the same way as we hyphenate "cake-eater": "ordinary-looking". The hyphenated form can be found online at Oxford Dictionaries.)
"Looking" can be modified by an adverbial adjunct, but as far as I know, it cannot take an adverb for a complement
As far as I know, there are no circumstances where the verb "looking" takes an adverb as its complement, but like any other verb it can be modified by an adverbial adjunct.
The characteristic feature of an "adjunct" is that when it is removed, the sentence should still make sense. So you can say "He ordinarily looks better than an average drow," where "ordinarily" is an adverbial adjunct and "better than this" is the complement of "looks"; the sentence still makes sense if the adjunct is removed ("He looks better than an average drow").
A sentence like "He ordinarily looks" or "He looks ordinarily" is problematic for the same reason that "He looks" is problematic: the verb "looks" lacks a complement, which often sounds unnatural (it can be acceptable in some contexts where it's possible to interpret it as an elision of a longer expression).