Etymonline has some interesting information on this:
late 13c., cobelere "one who mends shoes," of uncertain origin. It and cobble (v.) "evidently go together etymologically" [OED], but the historical record presents some difficulties.
Cobble is also of interest:
"to mend clumsily," late 15c., perhaps a back-formation from cobbler (n.1), or from cob, via a notion of lumps.
Etymonline defines 'cob' as:
a word or set of identical words with a wide range of meanings, many seeming to derive from notions of "heap, lump, rounded object," also "head" and its metaphoric extensions.
And ODO has 'heap, lump, rounded object'-like definitions for 'cob':
- the central cylindrical woody part of the maize ear to which the grains are attached.
- a round loaf of bread.
- a hazelnut or filbert.
- a roundish lump of coal.
- a mixture of compressed clay and straw used, especially in former times, for building walls.
As you can see, the etymology is uncertain. It could be something to do with lumping things together, but your guess is as good as any.
'Cobbler' is primarily used for shoes, as many definitions point out. (I can't find one otherwise, but that proves nothing)
Ergo, I'd say it's safe to assume 'cobbler' is of or pertaining to shoes, unless otherwise specified.