When the narrator refers to Ichabod Crane as a "worthy wight," the tone is undoubtedly mocking and ironic, for a few reasons.
Firstly, the use of the term wight is meant to evoke a sense of the archaic. From the vantage point of 1820, the narrator refers to 1790 as a "remote period of American history." On one hand, it is silly to think of thirty years as being so long an amount of time in a historical sense, especially because the events of that time would have still been well within living memory.
However, the narrator is poking fun at how young America is as a nation. Unlike Europe, which has centuries of history and folklore, the United States is not yet fifty years old at the time of the publication of Irving's story. Therefore, the term wight adds to the satirical sense of 1790 being a distant epoch from the perspective of the early nineteenth century.
Secondly, the word wight has two meanings, and both fit the character of Ichabod. The word can be used to describe ghosts or spirits, but its older meaning simply refers to a living creature, particularly an unfortunate one. Since "The Legend of Sleepy Hollow" is a gothic story dealing with the supernatural, the word wight conjures up such associations early on to help set the tone. Even when using wight in its older meaning, the narrator is on point, since Crane is indeed unfortunate regarding his proposal to Katrina and in his encounter with the Headless Horseman.
Thirdly and most obviously, calling Ichabod a "worthy wight" is amusing in that Ichabod is hardly anyone's definition of worthy. He beats his students, pursues a woman for her money alone, and harbors superstitious beliefs. He is a weak person overly influenced by the "old world" of Europe from which Washington Irving seems to believe the young United States must break in order to achieve its own identity. Even the term "worthy wight" evokes the phrase "worthy knight," suggesting old European tales of chivalry and romance. Once again, this is Irving gently suggesting that Americans must have their own national folklore and literature, such as "The Legend of Sleepy Hollow" itself.