Doubling the same Chinese character is one of the Chinese naming techniques. Doubling the same Chinese character is also used in order to bring out "cuteness". "MoMo" could be for a guy whose name is Moses or his last name is "Mo". To call someone with the doubling Chinese characters is normally for younger guy/girls; you normally don't call an old guy or lady using that. So I believe, they use Yoyo, Bobo, Momo, etc because they are considered cute and young.
I am not a historian or an expert in cultural differences; don't quote me. I am not here to argue, so it's ok if you disagree with me.