After seeing this scene many times, I have realised that Lamar's roast is actually incredible. By acting as if he's in a hurry and talking quickly, he eliminates the opportunity for Franklin to counterattack, meaning that he can continue delivering his verbal abuse.
The use of the phrase "yee-yee" is especially clever as it has very loose meanings and is utilised to further demean Franklin while not being necessarily rude, as well as fitting nicely into the rhythm of Lamar's "smack-talk" as they call it. Since a person's hair is usually one of their most stand-out and noticeable features, by ridiculing his, Lamar makes Franklin's identity feel attacked and being shorter means that he's opened to the perspective of one taller than himself having to look down at it, making him especially self-conscious.
Furthermore, by berating Franklin's female partner, Lamar starts to get very personal with him which makes the insults even harsher, while also making Franklin feel inadequate in romantic relationships. As well as this, feigning uncertainty of her secret partner's occupation and referring to contemporarily intelligent and respectable vocations such as "lawyer" and "brain surgeon" Lamar attacks Franklin's lack of a formal education and business smarts.
Finally, the strange and belittling way he says "nneEegaahh" further aims to make Franklin seems foolish and by slightly squatting to make eye contact, Lamar subtly roasts Franklin's stature again, as well as his status, making him feel like an insignificant child.
Truly, a genius in the art of "roasting" as those kids with their exaggerated swagger call it.
