Alex,
They're all actually rather clever. Tracy's bits are fairly awesome as she does incredible character bits and accents. I remember the first time I saw her doing an American accent, I was impressed. And the American black woman. It's spot on - and the character doesn't work unless she's black - plus, she's in brown makeup which is damned realistic.
The Little England Minstrels bit is priceless. It's lampooning a British version of Pauline Hanson and the joke, of course, is playing up on their blackness done in black face. They could have used, say, "mimes", but since they're lampooning a racist campaigner, it works just fine as humor.
The "Linda" bits are playing on the fact that she's a thick-skinned insensitive idiot. She offends everyone, whether it's racial or stature or looks.
In short, they all work as humor - humor by my tastes, at any rate, and I accept that that's a subjective thing. I'm politically left, but I'm a dedicated fan of satire, sendups, lampooning, what have you.