If it’s a pokémon with a reasonable enough level of intelligence to consent to being tattooed, it’s actually fine, believe it or not. (Unfortunately, this means you can’t tattoo a slowpoke.)
As for your question concerning what happens, because it’s a permanent change to your machop’s body, it will actually shift to an analogous spot post-evolution. For example, if you tattooed a machop’s bicep, the tattoo will remain on its bicep when it evolves to a machoke. Keep in mind, though, that the tattoo might warp as your pokémon’s size shifts. This might not be a problem for a single tattoo that won’t suffer from a little stretching, but a sleeve tattoo on a machop might be an entirely different story.
The Pokédex According to Bill 