If we're going to simplify it that far down the term loses all meaning though.
I mean, it's not a particularly deep or difficult narrative mechanic. A foil serves to highlight one aspect or another, and this highlights Luffy's willingness not to use violence just for the sake of pride. This was a pretty large plot point, given Shank's introduction that later carried to Luffy's confrontation with whathisname....Bellamy. Kidd would have never even thought of reacting to Bellamy's ridicule the same way Luffy did.
Whats the significance of this? I have no idea. But it's been a recurring plotline that Luffy just shrugs off those laughing at him, and Kidd specifically contrasts that.
That said, what makes BB a particularly great character, going by your metric? So he contrasts Luffy by being a piece of crap. So what? Why is this significant while Kidd being vicious is not?
Even I agree, what's the point of having two characters make Luffy more noble? Big Mom takes people's life force. Kaido slaughters entire cities/islands with his crew. How is Kidd unique? There is something unique about BB. But not Kidd. Every single one of these enemies show Luffy is noble. That's Luffy's entire MO - the noble pirate. How does this make Kidd special?
Because he's an up and coming rookie, like Luffy, part of the Eleven Supernovas. It's not merely being vicious, but he's specifically crafting his journey in a manner similar to Luffy's journey in the story, with the dividing issue being his viciousness, as opposed to Big Mom, whose viciousness is already in place before Luffy really even starts. What makes him a foil in particular isn't that he's just different from Luffy in that one regard, but that he's similar in nearly every other one.