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Minmaxing is fine, but the problem is people minmax Speed and a single Attack stat and completely miss the fucking point by doing so. Electivire is a common case. It's built to be a mixed attacker, but you always, ALWAYS see people say to give it more Speed and less Special Attack. Instead of playing to the Pokemons existing strengths and improving it there, they simply turn it into yet another fast, focused physical or special sweeper.
The rest are mostly right too assuming it's in the context most people do it in, which is just "give it things that other Pokemon already do well". If you just give everything Intimidate or Beast Boost then fuck off.
Making up an ability is fine so long as it is actually reasonable in terms of effect.
The question asks you to make THAT shitmon good. Not to turn it into a sweeper. Your goal when answering those threads should be to improve it within reasonable parameters, while staying true to what it's designed for already.