“A golden bit does not make a better horse.”

One application could be compared to materialism. The external display of quality does not necessarily demonstrate any inner quality. Politicians my display a taste for quality ice cream while inwardly, their intent is to reek havoc and mayhem.

I like Mike's approach to this one, but still, I have another approach that might be of interest.
My first interpretation is that Seneca pointed out how doing just a little bit of good, or a little bit of good work, or shining in a specific aspect, isn't enough. The horse that has a golden bit doesn't become a golden horse or a perfect ride. Similarly, we can't just say "I give some money to charity, so I'm a good person". It might not be enough. It might be just a golden appearance. We need to strive to be "golden" in everything we do, say, and think, instead.

Lot's of meaning here.  ;-)  Focusing on the word 'bit' — just because your jail cell has been decorated doesn't make you free.