Neither is really correct. The first one is definitely wrong. You will sometimes hear the second one used, but it is a slang form. The proper form is 'Don't pretend to be having a good time'.

Don't be pretending having a good time. INCORRECT Don't be pretending to have a good time. CORRECT. (It's not slang.) Don't pretend to be having a good time. CORRECT Eric Clapton song for your enjoyment: https://www.youtube.com/watch?v=x4ylS4vkfH8