What is the origin of "grapes" meaning the percent sign (%)?

The second commenter appears to be right. All mentions seem to link back to The New Hacker's Dictionary (The Jargon File) (1991), page 45:

• https://books.google.co.uk/books?id=g80P_4v4QbIC&pg=PA45
• https://archive.org/stream/jarg422/jarg422.txt

in which it takes up but a single-word mention, as one of a few names for the percent sign. That publication is the only one substantially mentioning "hacker" and "jargon" together and is likely where the Wikipedia editor got their edit.

First commenter is also likely right in that it has to do with the visual resemblance to a bunch of grapes.