In Harry Potter and the Prisoner of Azkaban we are told that the Riddikulus charm just changes the aspect of the Boggart and the consequent laughs are what make a Boggart disappear:
‘Riddikulus!’ he shouted, and they had a split second’s view of Snape in his lacy dress before Neville let out a great ‘Ha!’ of laughter, and the Boggart exploded, burst into a thousand tiny wisps of smoke, and was gone.
Harry Potter and the Prisoner of Azkaban, Chapter 7: The Boggart in the Wardrobe.
In Pottermore, the Riddikulus charm is described as:
Riddikulus is used to repel Boggarts. A simple charm, it requires force of mind to work successfully: the word alone is not enough.
However, in Harry Potter and the Goblet of Fire, Harry just uses the word (emphasis mine):
He summoned the happiest thought he could, concentrated with all his might on the thought of getting out of the maze and celebrating with Ron and Hermione, raised his wand and cried, ‘Expecto Patronum!’
A silver stag erupted from the end of Harry’s wand and galloped towards the Dementor, which fell back, and tripped over the hem of its robes … Harry had never seen a Dementor stumble.
‘Hang on!’ he shouted, advancing in the wake of his silver Patronus, ‘you’re a Boggart! Riddikulus!’
There was a loud crack, and the shape-shifter exploded in a wisp of smoke. The silver stag faded from sight.
Harry Potter and the Goblet of Fire, Chapter 31: The Third Task.
From what we read, it doesn't seem Harry had the time to think of a funny appearance a Dementor could take. More over, he doesn't laugh at the Boggart's new aspect, which is apparently a crucial requirement to make it explode in a wisp of smoke.
As pointed out by Himarm, there's another example of this Riddikulus' behaviour in Harry Potter and the Order of the Phoneix (emphasis mine):
‘Mrs Weasley, just get out of here!’ shouted Harry, staring down at his own dead body on the floor. ‘Let someone else –’ ‘What’s going on?’
Lupin had come running into the room, closely followed by Sirius, with Moody stumping along behind them. Lupin looked from Mrs Weasley to the dead Harry on the floor and seemed to understand in an instant. Pulling out his own wand, he said, very firmly and clearly:
Harry’s body vanished. A silvery orb hung in the air over the spot where it had lain. Lupin waved his wand once more and the orb vanished in a puff of smoke.
Harry Potter and the Order of the Phoneix, Chapter 9: The Woes of Mrs Weasly.
Is this just a plothole, or are Harry and Remus performing an advanced form of the Riddikulus charm (c.f. advanced Corporeal Patronus vs. standard lesser Patronus)?
I'm aware that @N Unnikrishnan addresses this (in my opinion) plothole in his answer to Casting “Riddikulus” on a Dementor?, but I'm not convinced by his reasoning; I'm interested in canon information, and educated speculations.