If John Thorpe is loud, rude, a bit dimwitted, blustering, and really overbearing – hilariously so, really. He's always yelling and swearing. He's even rude to his mother. He's also the world's worst conversationalist – all he talks about are horses and carriages. And to top all this off, John will also never ever in a million years admit that he is wrong about anything. So he's insufferable too.
This is a trait he shares with his dishonest sister, Isabella. Actually, these two siblings have an awful lot in common. OK, so Isabella doesn't yell and swear all the time. And she tends to manipulate others with more finesse. Isabella cajoles Catherine into doing her bidding. John, meanwhile, lies outright and at one point resorts to kidnapping. Kind of. In one scene he won't let Catherine out of his carriage, and refuses to admit that he lied about seeing the Tilneys in order to trick Catherine into going on an outing with him.
Catherine is upset by this, exclaiming, "'How could you say, you saw them driving out in a phaeton?' Thorpe defended himself very stoutly, declared he had never seen two men so much alike in his life, and would hardly give up the point of it having been Tilney himself." (11.53)
John is definitely more of an aggressive liar. But both of the Thorpes lie and scheme and manipulate others. In fact, John's lie, or exaggeration, to General Tilney sets in motion the events of the latter part of the book, where Catherine is invited to Northanger Abbey and is later tossed out by General Tilney.
Like Isabella, John functions as one of the principle trouble-makers in the book, making Catherine's life difficult. Of course, if John hadn't lied to General Tilney about Catherine's wealth, Catherine would not have been able to spend nearly so much time with Henry. So John is actually inadvertently helpful – something he probably would not appreciate, especially since he didn't exactly help himself there at all.
Since Northanger Abbey loves its parallel constructions, John contrasts with the book's other brothers, James Moreland and Henry Tilney, and with Catherine's other suitor, Henry. Unlike James and Henry, John is rude, loud, obnoxious, and aggressive. So, aside from playing a crucial role in moving the plot along, John also throws the good character traits of Henry and James into sharper relief by behaving so abominably.
John does pull a disappearing act in the latter part of the book. In fact, after Isabella and Catherine discuss the mistaken engagement debacle, John is not even mentioned again. Is it because he was no longer needed to make Henry look good, since General Tilney seemed to be accomplishing that? Does General Tilney effectively supplant John in the pivotal role of "person who causes Catherine lots of trouble"? At any rate, John is quite important, and even unintentionally helpful.