The substance should be the reason Trump is rejected as beyond the pale by anyone with a bit of common decency. But this article makes his style the problem, as if Trump would be a perfectly okay chap if he just ran his segregated housing politely or something. It's smarmy.
I think, if we were to be fair, if he were to become the Republican Nominee, he would suddenly become the target of much deeper scrutiny to the point where it'd be uncomfortable. Part of the reason no one is talking about those things is for the exact reason you mentioned, he's a clown, a joke. There's no reason to take him seriously. Chris Christie or Scott Walker on the other hand? People do take them seriously and instead of saying "Oh, those silly buffoons" people are saying "Oh Lord Mercy, look how these men have gone and fucked up as governors."