Twain uses dead-pan humor to create irony in this episode. Huck's narrative depiction of Pap's behavior suggests that this behavior may be normal. Huck does not offer commentary or editorialize Pap's antics at all and simply conveys Pap's actions in a matter-of-fact tone.
However, Pap's behavior is not normal. Having climbed in a window and derided his son's ability to read while also attempting an obvious trick, Pap's behavior is far from normal and far from mature.
The discrepancy between Huck's matter-of-fact tone and Pap's outlandish behavior creates two readings of the episode. The presence of two readings equates to irony. (Irony occurs when there are multiple meanings in a given text or textual moment.)