BBC code works in your signature and elsewhere but you need to insert it manually.
Unlike in the regular post, there aren't any shortcuts to press.
Maybe you should use a regular post to see what the smiley's require.

A : followed by a ) would show up as

but you have to do it manually.
That also applies to bold text or larger text etc. and remember, all that coding eats into
the total number of letters you may use in your signature.