To put lipstick on a pig means you are making superficial or cosmetic changes to something that is fundamentally broken or unattractive, to make it more appealing. It likens it to putting lipstick, something that would normally make people more attractive, onto a pig which is generally seen as messy and unattractive.
Why are you building such a fancy porch on your old house? That's like putting lipstick on a pig!