Justify Mark Twain as a humorist in the context of this novel?
Hmm. What is the question? Is the question "How would I justify Twain as a humorist in the context of Tom Sawyer?"
If so, I'd say that several elements of the book justify Twain's reputation as a humorist:
-his ironic tone
-his observations of childhood activities, which blend affection with cynicism
-his control of dramatic structure
-his awareness of how people actually speak
(The first can be seen from the preface on; the last three are visible in the opening lines of Chapter 1.)