Just to put my thoughts on this from my perspectives.
Is Gotenks so universally hated as Kefla? If not, why? Isn't he also acting like an arrogant spoiled brat? Didn't he also effortlessly reached the Big Bad's power level without actually breaking a sweat? So why is Kefla any different?
Part of the reason is because Goten's a literal copy paste of Goku in terms of design and Kid ver of Trunks who I already liked since the Cell arc, to top that they're the sons of the 2 big badasses of the series, thats not to say I liked Kid Trunks, I hate his ass, Goten's a meh, and I'm not a fan of Gotenks but I can let it slide due to the reasons I said above (and this was before finding out that half breeds are supposed to be strong) and the next is.. they are literally physically small kids which is a norm when it comes to being a brat
Why was 18 loved for kicking Vegeta's ass but Caulifla is hated for (somewhat) giving Goku a run for his money?
Unlike a certain noisy cocky all over your face brat, 18 was chill and cold while whoopin ass. One of the problems I have with those saiyan girls is how they leap through powers, its like they skipped a lot of stuffs, I would have been fine if they were already strong and already had SSJ forms prior, but no, they had to show them getting form after form in a short amount of time with little to no effort and some sisterly love. God kill me.
Why was Vegeta praised for fighting Frieza back on Namek despite doing the same things Caulifla did (praising his own self while doing absolutely nothing to hurt the Big Bad)?
Because Vegeta looked like a scrub and even died in the process (badass prince even cried) hence the backlash of actually liking him, unlike Cauli who looked good fighting someone she should be leagues below and still eats her shit like "I would have won that, you just got lucky!" when Vegeta gets like that he normally gets the beating of his lifetime hence the people sympathize.
An example is Ribrianne, I dont like her yeah, but I learned to like her after her loss, she took it like a champ, thats how you earn respect and like from viewers.