Think theres a fair bit of overinterpretation going on here. The calf-roping remark is areference to the cowboys around the pool table who, as the screenplay says, got piggin strings looped on their shoulders. Piggin strings are used by calf ropers and it would stand to reason that a rodeo clown would be freinds with tha calf-ropers, as during that event, the clown would not be working (no need for him there) and by extension likely be off-duty and b.s.ing with the ropers during their event. Dring the bullriding and bronc events, there is the need for the clowns' distraction, so the relationship would more likely be a purely working one.
My interpretation is that this clown, for some reason, is uneased by Jacks overeager freindliness and maybe senses theres something under it, or maybe he just looks down on Jack as a loser, but in anycase, he rejects his offer of a drink and goes back to his buddies around the pool table. Jack looks back and sees them talking, maybe Jimbo is telling them about this queer trying and hit on me, maybe he forgot all about Jack soona s he turned his back on him.
Then the bartender makes his comment, and it could be a "moving right along" subject-changer like "so how bout those Longhorns" or it could be a suggestion that if Jack wants to get in with Jimbo, he should take up the same event as his buddies, or it could also be seen as an underhanded comment on Jacks lack of success (that eveing, anyways) as a bullrider and a suggestion he take up the less dangerous (an maybe implicitly) less manly sport of calf-roping.
Jack doesnt know, but hes embarrassed at being shot down, and also suddenly hes paranoid, thinkin everyone in the place is whispering about him. I heard there was an different ending to this scene where its more clear that Jimbo "outs" Jack and outside the bar, the calf-ropers are jeering and hassling him, and I think the ambiguity is better. Your not exactly sure whats going down and neither is Jack, and this highlights his potential vulnerabilty, if he was to be caught out in the open, as well as giving the truth to Ennis fears on their making a life together. The risks were real and could boil up from nowhere. Therer are potential enemies all around.