Brendan Moody wrote:
It's never explicitly said in the books who is responsible for the attempt on Tyrion. He assumes it was Cersei, but is still looking for concrete evidence when other events make the whole matter irrelevant. But the show flat-out saying at this point that it was Joffrey is certainly another case of making her nicer and him (even) nastier.
The show doesn't flat-out say anything about that. Cersei doesn't answer and Tyrion just assumes
that it must've been Joff.