There are two meanings of "good looking". One is aesthetic, where it means pleasing to the eye, and there is no doubt that some guns, like some cars, tablewear, clothing, etc., are more pleasing to the eye than others, but it is largely subjective. The other meaning is condition. A beat-up, rusted junker is not as "good looking" as the same gun in new condition.
The first meaning will not affect value, nor will it affect function. An "ugly" gun (in my opinion) might be in excellent condition, be accurate, and function perfectly. Since the gun is what it is in regard to its "beauty" (or lack of it), that will not affect the value.
But the condition will affect value, a lot. Even a gun most of us would agree is "good looking" in the first meaning will have little value if it is in poor condition.
Functionally, "good looking" in either meaning has little effect. If a gun works as it should, and is reliable, appearance matters little. In fact, some people who carry guns prefer one in only so-so or even poor condition if it is reliable because they feel that if they have to use it, losing a clunker to a police evidence room is better than losing a "beauty".
Jim