Kunk Kunk felt to me like Peter Dinklage speaking rather than Tyrion. The mockery was out of character; considering Tyrion loves 'cripples, bastards, and broken things', finding his "moron" cousin so fascinating that he apparently spent a significant amount of time just watching him punch dirt doesn't really reflect what we know about Tyrion's upbringing. In the end it felt like he was full of shit and he had just invented his cousin on the spot to make a parallel to The Mountain's specific brand of chaos.
He's then immediately handwaved away in the laziest way possible:
"Whatever happened to our dear cousin?"
"Oh he died when a horse kicked him in the chest."
"LOL."
For Tyrion who just came away from a court scene where he roasts everyone for judging him because of the way he was born, it's a little hypocritical to turn around and muse about a mentally handicapped relative.