I think that depends on the objective and how experienced they are as a martial artist. Joints are quite small and if you don't aim properly they could potentially hurt themselves. It's also possible that people could use the fact that your aiming at their joints to make you hurt instead. Let me tell you from experience that kicking someone's elbows when they are ready for it hurts like hell.
However there's little to no way for someone to strengthen that part of their body so if your aim is to distract them for a little while or create an opening for another attack or run away aiming for the crotch is actually a good method especially for those inexperience in fighting
The final page of the chapter killed me.
We can see that it is thanks to the hard training of the grandmother that the protagonist has become so strong. 🤣
@arashikitsu you forget, that grandmother is a master martial artist/warrior/fighter and training her granddaughter as such so she shouldn't be giving that terrible advice.
@TheGodEmperor The "aim for the crotch" was apparently said as a reply to whether it was ok to aim for the face (apparently a thing used to scare female fighters) and granma's advice should probably be taken in that light. I.e. "if they try to scare you by threatening their face, you can probably get them to react by threatening their balls" - then once they start defending their balls, you get easier access to their joints or whatever you really want to hit to end the fight.
(I.e. "don't hit it, just aim for it" a.k.a. faint/scare?)