I'm not sure it's true that bigots are necessarily low-grade people

I read the chapter much more narrowly as saying that racist people are low-status. Racism is now reviled in Britain (or at least the U.S., and I'll guess also Britain) to such a degree that anybody openly espousing racist views (at least based on skin colour rather than, say, immigration status) is automatically looked down upon. Other forms of bigotry don't usually have this effect, nor did racism until fairly recently.

However, we are looking more at racism in the 1940s (at least by the standards of the U.S.) than the 1990s. Judging from To Kill a Mockingbird (which is the only documentary evidence that I have onhand, sorry), extreme overt racism along the lines of using words like ‘Nigger’ (analogue of ‘Mudblood') was still looked down upon and associated (rightly or wrongly) with low class. But that's just because moderate and subtle racism was the norm. This is how it works in the Wizarding world too.