@Dzongkal: Sandy does not have The Three Ss* of what leads to characters getting a lot of fan art: Shipping, Setting, Sex appeal.

Shipping: Sandy lacks any super-viable shipping targets, since all the other core cast members are... well they're SpongeBob, Squidward, Patrick, and Krabs.

Setting: Bikini Bottom isn't really that interesting of a setting for fan artists to explore. its kind of, I dunno, "flat", it really does not have much of an identity; it's whatever the show needs it to be and after that it's just <suburban town>.

Sex appeal: most of the time Sandy is depicted in her giant, unflattering space suit. she does get a few points in this category for the bikini she wears while in her house, but it's just not the main outfit associated with the character.

_{*something I made up just now, not a real thing.}