It is not supposed to be insulting. According to Wikipedia:
To put “lipstick on a pig” is a rhetorical expression, used to convey the message that making superficial or cosmetic changes is a futile attempt to disguise the true nature of a product or person.
Though, it may not be conveying my idea. Something like “you cannot teach old dog new tricks” might have been better, but not exactly what I wanted to say, either.