Rin Tohsaka gets the benefit of us seeing things from her perspective in the prologue and Bulma is a comedic character whose meant to be a little insufferable so it works.
Rin Tohsaka is one of those characters who primarily get away with being a Tsundere due to looking at the Prologue, her situation and the fact that she literally has to put up a mask because of what she got from her father... and it actually does fuck her over in some of the Bad Ends due to that mask(including getting the absolute worst end due to that).
It does make it better that Nasu and the other writers tend to literally have Rin's more nebulous actions tend to backfire not just because of her good nature... but because she literally has a family issue where apparently all Tohsakas fuck up at the finish line.