Answer: It is ridiculing and despising one's Muslim brother, which is impermissible.

Forbidding mockery, Allah Almighty says: {O you who believe, let not some men ridicule others, for it may be that they are better than them; nor let some women ridicule others, for it may be that they are better than them. Do not speak ill of one another, nor call one another by [offensive] nicknames. How evil is the name of wickedness after having faith! And whoever does not repent, it is they who are the wrongdoers.} [Surat al-Hujurāt: 11]