I had to fix the title for accuracy. (S)he didn't quite endorse him but did speak well of much of his platform.
Jenner's known to be a Republican. I think that most people, although they may not agree with it, realize that Jenner's free to do whatever (s)he wants to his(her) body. It's not like people are going to say "Ew. Bruce Jenner endorsed Cruz! All the better to support TRUMP! Real women love him!" It's not really relevant to most people.
In contrast, David Duke is toxic, as he represents white supremacy, a "wrong thought" in modern society. Transgenderism is not a "wrong thought."