working class/poor does not, automatically = poor spelling, poor grammar.
BUT
using it to attack/laugh at others does = nobbish.
Why? Because our language is still used as marker of access to power and privilege - a quick signifier. And that means the ability to use "proper" English, when you want to, is both a useful tool, and saturated with power.
One of the reasons it is such a power-saturated signifier of access to power and privilege has to be because access to the means to learn "proper" English is limited, and is also power-saturated.
Yes, the above is a huge simplification. And I'm sure that someone is going to come on and give the usual thing about "well, the answer is that we should learn to embrace all forms of English". And that, of course, is true. But it is also true, that the liberals among us (such as myself) can do all the embracing they want, but still, there is a huge power issue involved in access to knowing different forms of English, all of them, including the high-status ones, and being able to use them in appropriate situations.
So, to go back to the earlier point - laughing, just mocking poor spelling and grammar is not OK, because you really are mocking disadvantage. Though surreptitiously.
That doesn't mean you have to love it.