Who is Pancakes?

Currently, there is no reliable information or source about the tombstone inscription or its meaning.

Some people pointed out that the inscription is an anagram for "A Epic Prank" (however, missing the letter s), which could be seen as a reference to Trolling is a art: The purpose of this phrase is to troll people by getting them to correct it (example).

A collection of additional fun facts/easter eggs regarding Whimsyshire can be found here and here.

A little OT: Blizzard put some stuff into the game where the purpose of is not known by now, e.g. the Black Rock Ledger. There exist long discussions about it. Perhaps the tombstone will also turn out to be another clue to something bigger.