The scene in question can be found here. Transcript:
Randall: 'Wazowski! Where is it you little one-eyed cretin?'
Mike: 'Okay, first of all it's "Cretan", if you're going to threaten me, do it properly.'
But cretin is the commonly used insult, and it's not mispronounced. Via Google:
a stupid person (used as a general term of abuse).
a person who is physically deformed and has learning difficulties because of congenital thyroid deficiency.
cretin is sometimes confused with cretan
relating to or characteristic of the Greek island of Crete or its inhabitants.
"ancient Cretan pottery"
noun: Cretan; plural noun: Cretans
a native or inhabitant of the Greek island of Crete.
"on the road he meets a Cretan"
I was surprised to find zero commentary on this online given it's a 19 year old film, albeit a rather minor part of one. Google notes that the two words are commonly confused for one another given their relatively similar pronunciation, but considering that Mike corrects him I assume there's some intentional joke here - but I can't figure out for the life of me what it is. Given Crete is a Greek Island I assumed it might have something to do with Greek mythology but I can see nothing else that might back this up, Crete, unfortunately, has nothing to with Cyclopses, though I almost want to pretend it does and that this is a great, obscure joke.
I only actually came to rewatch this scene after a friend, who defended the use of cretin in a discussion of whether it should be censored from a video we were making on the basis, mentioned that it was used in the film Monsters Inc, but after a while, I noticed her pronunciation of the word was odd. It would be interesting to see whether this scene actually influenced people's pronunciation of the word, but that's a separate matter.