Once again, you are applying your own set of values to something that is completely subjective. Roleplaying = acting, and by definition that is a completely personal expression of how the actor believes the char should be portrayed in that particular setting.
Once again, you are failing to misunderstand the basic premise of the original post.
But to use your example: an actor can portray a character however they feel like it. However, the portrayal may be an offensive mockery.
If you literally don't know if something is an offensive mockery, then try to find out.
For example, go watch The Producers (the original movie). What the producers thought would be a offensive disaster ended up being a hit, as the audience loved it. I don't know your what your thresholds are for what is being a "decent person", and what is "being a jerk". And you don't know mine. And you don't know anyone else's either, unless they precisely tell you what they found offensive, and why.
Are you deliberately trying to make fun of someone or a group of people by mocking how they act or how you think they act? Then you're quite probably being a jerk. This is particularly the case if you're mocking someone for an inborn trait (intelligence) rather than a choice they make (political party).
(Also, you do realize that the Producers is a movie and the audience in it was
written to find it funny. From what I've read, the actual movie had mixed reviews upon release but gained popularity later on.)
Every single person is different, and no one has the right to say to another person "You have to stop that behaviour, because I find it offensive" Now, if you want to get a group of people that agree with you that some precise act or comment is offensive, then yeah, we see now every day how that works. But as an individual, no.
Everyone has the right to say that to other people. People aren't required to
stop the behavior--but on the other hand, if someone is telling you they find your behavior offensive, then it might be a good idea to examine the behavior in question. Maybe that person who told you that is being a jerk themselves. But maybe
you are the one being a jerk.
(Freedom of speech is not freedom from consequences.)
And again, it heavily depends on whether the behavior is an inborn trait. Ones ethnicity, sexuality, gender, raw intelligence, etc. is an inborn trait, and shouldn't be discriminated against. Being a bigot, however, is not inborn.
So, how about this:
I am playing a Paladin with a dump stat of Int = 8. (I never play, nor allow chars at my table with stats less than 8).
The Paladin says to the group "You guys with your puzzle-solving brains, too much for me, wake me when you are done". Offensive to you?
No. Because you're not making fun of people who aren't smart.
Also, you don't allow low stats? That's a shame. There's a player in one of the games I'm running with a 6 Wis warlock and he's an absolutely awesome character. Tons of fun to DM for.
How about the same Paladin when faced with doing some mathematical calculation really quickly, like calculating the volume and mass of a huge jug of water, in order to rescue the party from a trap, and the player says, in-game "I am not smart enough to do that!" Offensive to you?
No, because you're not making fun of people who aren't smart.
Waaay back in the 80s, it was common for the kids at my school to call people stupid by saying their name and making an arm gesture that--although I didn't realize it at the time--mimicked the muscle contractures of a person with cerebral palsy. You know: arm held up with a loose wrist and the fingers curled under. According to
this site, about half of people with cerebral palsy have intellectual disabilities, although I'm sure the kids at my school assumed it was always the case, and they probably didn't even know what it actually was, just that "dumb people" held their arms like that.
So if you decided to play a low-Int character and used a gesture like that to indicate when you were speaking in character, I think just about everyone would say you were being offensive. It's unlikely you're
that talented an actor to pull it off, and D&D isn't really built to stat out specific disabilities like CP.
In reality, though, I'd bet most people who are going to be playing a "stupid" character are actually doing lolrandom stuff like going out of their way to deliberately trigger traps or pour that full jug of water over the angry archmage the party was negotiating with because "that's what stupid people are like." And this is
also offensive, and disruptive to boot.