I don't normally mind slapstick too much, HOWEVER, I do not like it in this case because Minami was clearly in a very physically abusive environment, but Iori keeps hitting her. That is not okay with me.
I kind of got the impression it merely mildly annoys Minami, who after all has ample experience with serious violence and fights. Iori, a complete amateur and not exactly a physically formidable specimen, hitting her probably works about the same as a kitten attacking a scarred alley cat - she isn't taken seriously at all. (Minami herself will also dope slap Iori for trolling, FWIW.)
...then again if you look at how her first meeting with Saori - a trained fighter who can and does hit very hard when she wants to and casually made threats against her life - went Minami probably also has some unhealthy issues about that kind of thing, given she apparently found her adorable... >_>
Shizuku and Saori have a very different dynamic which, if I'm reading it correctly, basically boils down to the former still being too damaged to quite function normally and the latter being able and willing to face her frequent relapses into violent outbursts head on. Saori being perfectly capable of beating her down probably also has the side effect of establishing her as the "dominant alpha" in the rather primitive mindset Shizuku seems to operate on - that is, someone worth respect and listening to. They also have an unspoken agreement going that while Shizuku lashing out at Saori is duly settled with fisticuffs and no harm no foul, everyone else in the school is Right Out - the one time Shizuku broke this, even if for arguably noble reasons, Saori got hella upset. (Not in the least, one imagines, because she understands what kind of scrutiny and prejudice Shizuku is subject to due to her ugly history - lashing out at "civilians" will harm herself.)
Rather messy that whole thing, but it does serve to effectively contain Shizuku's difficulties in adjusting to civilised society.