CGDAE makes more musical sense, but you can't say it so it isn't a cute mneumonic.
Open strings are
EADGBE ...it's inconvenient to play a key that puts and accidental on any of the open strings. Let's use that criteria for: a good indicator of the most popular keys. But we need to use sensible music theory instead of a mneumonic to get the answer. Assess key signatures in relation to the open strings.
Go the way of the flats first:
Bb stop! You hit one on the first key signature with a flat. Keys with flats are inconvenient on guitar standard tuning.
G# stop! Up to two sharps is OK. After that you can't use the open strings for key signatures with 3 or more sharps.
CGDAE disorganized as
CAGED. Start with
C using zero sharps and flats then go up (in fifths, hence my complaint about the order of letters in
D is the key signature with 2 sharps. So,
D use all the open strings which makes them pretty convenient keys to play on standard tuning.
C-G-D is what you are left with from the word "caged."
CGD and not
Not much of a mneumonic, but you don't need a mneumonic with a sound grasp of the core theory of key signatures and musical spellings in perfect fifths.
You might ask further: why not
F just as inconvenient as
A in that both are only one accidental past the open strings? The trouble with
F or any of the flat key signatures is that you need to lower the
Bb. How do you lower an open
B string? Of course you can't. You have to skip using the open
B string and fret it somewhere else. By comparison adding sharp keys to
E involves just fretting one of the open strings on the first fret which isn't as convenient as playing all open strings, but a lot easier than dealing with flats.
Maybe you can fudge things a bit and use lowercase to indicate minor keys for
e minor. Those key signatures are of course zero sharps and flats and one sharp respectively.
CGDae or camelcase
CaGeD! Actually, that isn't half bad as the relative major/minor keys are paired up!
CaGeD, but pronounced with soft A and hard G, like "kah-ghed" :-)