Handwriting is to illegible as drawing is to what?

I would say the drawing is unrecognizable:

un·rec·og·niz·a·ble (adjective) — not able to be recognized or identified.

You might say that the drawing is "indecipherable", which is defined by Merriam-Webster as "impossible to read or understand : not decipherable".

I would go for inscrutable. There are many words that do the trick, and a lot of good answers, but I think inscrutable works particularly well. In fact, the first example in Webster's definition refers specifically to art!

inscrutable

adjective

\in-ˈskrü-tə-bəl\

not readily investigated, interpreted, or understood

an inscrutable work of art

Source: Merriam-Webster.com

Oxford dictionary has this to say:

"inscrutable. impossible to understand or interpret"

Oxford dictionary, Mac edition

Or, perhaps, unintelligible, which works for representational art and might also work if one were referring to non-representational art.

I would personally go for

Incomprehensible

Adjective, not able to be understood; not intelligible.