Why do you find it in interesting that someone chose something mainstream? Isn't that the definition of mainstream, that it's a common choice?
If OP got a tattoo of Pikachu, it was interesting to me that he picked the most mainstream pokemon (I assumed he was a huge fan)
But from the response, my sample of 1 assumption checks out: "When I was planning it was obvious there'd be a Pokémon due to his (continuing) interest in those and to me as a non-fan that's the most immediately-recognizable one."