Bcs ... if its behaviour, i dont know what kind of pixies you know, but here in mid-Europe pixies are known as the worst pins in the ass that ever not even existed.

I know pixies from forgotten realms lore - you know, the type of pixie we're talking about? Specifically
not your mid-euorpean earth pixie?
This is kind of the complaint that I'm highlighting:
In
Our World, in a particular cultures, pixies have a certain cultural history and lore. That's great.
It's Also
Not The Forgotten Realms. It's entirely irrelevant what your cultural representation of a creature is like when that is
not the creature we're talking about at all.
Larian have seem to have gone "We know what pixies are; our cultural history tells us what pixies are!" and haven't bothered to check whether their concept of pixies as they know it from their own real-world cultural mythos lines up at all with the completely independent fictional creature that exists in the forgotten realms and has its own lore; hot tip - it doesn't! They did the same with vampires! Wooden stakes through the heart do not kill vampires or vampire spawn! Sorry, not this mythos, not this lore.
To put it simply:
No. They
Don't be like that. Quite specifically,
these fey do not, and are not, like that. It's part of their intrinsic nature (not even inherent nature - actual intrinsic nature) that they are not malicious creatures; they literally do not have the capacity for it. (Unlike mortal creatures who have the capacity to direct their alignment and morality, intrinsically good creatures - as pixies are - don't.)
If they wanted a corrupted pixie, capable of malice, then the successful check should have revealed
that, instead of just saying that pixies can be malicious (they can't).
If they wanted a creature that is small, mischievous, with the capacity and potential for great malice as well, then they could have made the tiny creature a quickling.
My complaint is not that the pixie was grumpy - though the crudeness was unnecessary and I don't feel the crude humour added anything of value to the scene, while detracting from it actively - my main complaint is that the check that the character made was successful and provided abjectly false information in order to sow doubt falsely.