Word for damage that isn't serious, mostly visual and not hindering the function of the thing

Solution 1:

Such damage is sometimes referred to as "cosmetic". Oxforddictionaries.com defines this sense of the word as "affecting only the appearance of something rather than its substance".

Solution 2:

Superficial is the word you're looking for.

[Merriam Webster]

: concerned only with what is obvious or apparent : not thorough or complete

: affecting only the outer part or surface of something : not deep or serious

"the damage was superficial"

Solution 3:

I've heard this called, just a flesh wound. See also: but a scratch.

From the source:
King Arthur: [after Arthur's cut off both of the Black Knight's arms] Look, you stupid bastard, you've got no arms left!
Black Knight: Yes I have.
King Arthur: Look!
Black Knight: It's just a flesh wound.

source: Monty Python And The Holy Grail - Just A Flesh Wound