Final answer:
The specific reason Heron mocks Byron is not stated, but it may relate to Byron's controversial reputation and political stances, such as his support for Greek independence.
Step-by-step explanation:
The reason Heron mocks Byron, who Stephen says is the best poet, is not directly stated in the question or provided information. However, based on historical context, one might infer that it could be tied to Byron's reputation as well as the themes of his poetry which sometimes clashed with the viewpoints of those like Heron. Byron was indeed a controversial figure – his lifestyle and beliefs, including his criticism of English society and politics, as well as his support for causes like Greek independence, often drew ire and mockery from peers who disagreed with him politically or morally.