Jotaro got his ass kicked by a rat because halfway through the fight he realized he wouldn’t have to write a thesis if he was dead and he weighed his options
The thing to remember when watching Stardust Crusaders is that Jotaro is 17. He’s a child. Keeping that in mind, everything about him is funny.
He goes around scowling and saying rude one-liners like he thinks he’s fucking Shadow the Hedgehog or something.
He has a catchphrase.
Said catchphrase is a statement of being bored/exasperated which is often translated as “good grief”.
Everything he does is fucking extra for the sake of looking cool, like in the N’Doul fight when the jeep flips and he’s hanging on with one hand with the other in his pocket.
His outfit is a school uniform that’s been heavily tailored and accessorized. That fucking big-ass chain on the collar. And he liked the way it looked so much that he continues to only ever wear variations of that same outfit even when he’s ~40.
His solution to every problem is to punch it, and if that doesn’t work he either punches harder or throws something.
Star Platinum’s “ora ora” battle cry must have came from somewhere since Jotaro fucking says it himself when beating up Alessi as a 5 year old with no recent memories. So he’s probably copying some anime he saw as a little kid because he thought it looked badass.
The fact that he has such absurd analytical skills and he went on to become a PhD implies that his entire persona is a front for being a fucking nerd. And as Stone Ocean teaches us, Jotaro is genuinely awful with interpersonal relationships. In other words, SDC!Jotaro is a nerd with poor social aptitude acting hard and edgy to cover this up.
He faced down a megalomaniacal vampire with the power to stop time and a personal vendetta against his entire family, and killed him, while putting on airs so nobody notices how socially awkward he is.