Given the jokes against blacks and Africans throughout.
If it were white people saying things like "wearing clothes must be a new experience for you," we'd have people claiming racist jokes.
Those lines were designed to make the character a bigot, regardless of skin colour. But, yeah, if somebody white played the boyfriend it would be inexplicable as to why Lisa wouldn't tear him to shreds right there.
I don't think the main cast had to be all-black, but I don't see why they shouldn't have been. The actors were all well-suited to their roles. John Amos was freakin' hilarious.