Is there an equivalent of 'onomatopoeia' for words inspired by the appearance of thing?
Solution 1:
An ideophone — a word that sounds like its idea or concept.
It's quite a general term that could include onomatopoeia. There are other terms for certain types of sound symbolism.
Solution 2:
John Lawler has some works on phonosemantics, which connect the meaning of words to the way they sound. I think that Bouba/Kiki, onomatopoeia and the OP's question can be seen as special cases of phonosemantics.
Perhaps @JohnLawler himself can drop by and explain much better than me. :-)