Yes, I agree with you about the presence of fear and the need for fear and caution due to the lingering prevalence of hatred and homophobia in our society. And I do not doubt that Jack inevitably met his end due to prejudice and irrational hatred. Although there is even ambiguity about this...was the killing of Jack just a figment of Ennis' imagination? We do not know. It never says for sure. We are left to make our own conclusions based on our prejudices, beliefs and outlook of the world and people's natures.
Getting back to Jimbo however, there is another reason for the rodeo clown scene. In classic stories of all kinds, the figure of the clown often appears. On one level, the clown is seen as an escape from the serious side of life, but on another level, the clown is a symbol of randomness and chaos. The clown can even be menacing, because through one misstep, your life can be taken away and life goes on without you, uncaring, unknowing. The fool or the clown is a fixture of Greek tragedies, Fellini movies, as well as Batman movies. There is a double dose of clowns in The Dark Knight, since the movie begins with the Joker in a clown mask leading a bank heist. The director, Chistopher Nolan, is one of the most existential auteurs directing today, so this movie is revealing of our culture.
Another representation of the clown is the Devil, who appears in a couple of Annie Proulx's latest stories in her collection Fine Just the Way it Is.