I think the point is not that you shouldn't decorate your gear, but that you should be aware of the image you project. For the "serious" musician (a term I am not in favour of, other than for describing po-faced old fuddy-duddies) playing in an orchestral context, then yes, probably evening dress and a pristine instrument is the way to go.
And yes, if you stick a candelabra on your instrument, the audience is going to anticipate Liberace.
For what I do the industrial/DIY ethic dictates an oh-so-carefully-cultured function-over-form look with hulking great visually mismatched amps and a beat-up theremin with gaffer tape and scratches. One has to pay so much attention to looking like you just don't care. ;-)