>The mainstream parties are not offering alternatives,

This part rings a little hollow when it's one of those mainstream parties that's doing the demagoguery.

>can you blame those who are disaffected by this rapid societal change from reaching out to support the first name that voices critique at policies they dislike?

Yes and no.

Can I blame people for recognizing that something in their life isn't working? No. Can I blame them for willfully accepting claims that offer no attempt at proving themselves with evidence, and that consist almost entirely of "people who look different than you are the only reason you have any problems"? Yes, because the average person shouldn't be so easily duped, and if they are, it's generally because they already wanted a reason to blame "those people" and anyone offering the faintest excuse ought not be "good enough".