Harry Potter is a high-magic setting and magic is everywhere. There are very few limits on what spellcasting can accomplish, given you know the right spell. (Some of those limits are Gamp's Elemental Laws of Transfiguration or the fact that you cannot return someone from the dead, but we don't get a lot of them. Felix Felicis, the Philosopher's Stone and Time-Turners give their users terrifying power over causality, mortality and time itself, respectively.) New spells and potions are invented all the time. Magical beings range from boring Flubberworms to square-cube-law-shattering dragons, on which there don't seem to be firm limits for what is "plausible" or not.
Yet when we are introduced to Luna Lovegood at the beginning of OotP, Hermione pooh-poohs the Quibbler as a conspiracy rag. It claims the Crumple-Horned Snorkack is real! It claims Cornelius Fudge has a battalion of Heliopaths and is pursuing a secret campaign of goblin genocide! How absurd.
Still disparaging the ridiculous idea of a Crumple-Horned Snorkack, Hermione climbs into a carriage pulled by carnivorous skeletal pegasi invisible to those who haven't seen death, and makes her way to the castle.
What heuristic could Hermione (and the wizarding world at large) possibly use to evaluate the Quibbler as ridiculous or not?
In a world where criminals (and enemies of the government) are sent to an island prison guarded by floating spirits of despair, why is the idea of a secret government battalion of fire spirits so silly?
In a world where prejudice against "subhumans" is depressingly widespread (as Hermione well knows) and relations with goblins remain strained since the bloody Goblin Uprising of 17-whatever, why is secret goblin murder so absurd? Especially when Hermione knows Fudge to be pursuing a disinformation campaign against Dumbledore and Harry, and generally trusts the Ministry as far as she can throw it?
In our world we develop heuristics, based on our own experience and what we are told, to sort things into "believable" and "not believable." I know the square-cube law makes big animals cumbersome, and the biggest flying animal I've ever seen is a condor, so if you told me you saw elephants flying the other day I would be disinclined to believe you. Conversely, I've ridden in airplanes before and have a rough idea of how they work, so the idea of a supersonic plane like the Concorde doesn't weird me out.
The thing is, I don't see why someone growing up in the wizarding world would draw the line at anything. Students at Hogwarts are exposed to new and wondrous spells, potions and creatures all the time, and the apparent limits of possibility are pushed further with every passing school year. So what gives?