If they meant it, then yes. Most often (and slightly more on topic for this site, since it does actually relate to the use of the English language), it's not a fallacy, but a rhetorical technique.
If I reason from the grammatical error, then (barring some cases where e.g. the person is claiming to be speaking from expert knowledge of grammar) that is likely a logical fallacy (though see Douglas Neil Walton and Charles Taylor for cases were ad hominem reasoning is not fallacious).
If I argue from the error though it might be because of that, or it might just be rhetorical technique where I don't necessarily believe it demonstrates my point, but it may convince others. This rhetorical technique is also called ad hominem, and in fact the term was used for this rhetorical technique before it was used as the logical fallacy.
I might also not expect readers to not believe my argument, but for it to still work. This is a combination of two rhetorical techniques; ad hominem and humour (or ioci if you really like your rhetorical techniques to have Latin names): I don't actually make an argument but I (or I do, but I also...) make fun of the opponent and so increase support for my position.
This is also often combined with a rhetorical stance, that doesn't have a Latin name, as far as I know, called "not putting up with this shit any more".
As I said above, referencing irrelevant features of someone's general stance isn't always a fallacy (especially if not taken as sole evidence), so all three can coexist. E.g. when the English Defence League tweeted "who's street's", then of course many of us pointed out the grammatical error. This is a combination of:
- Ironic humour; they're called the "English Defence League" and literally can't write two words of grammatical English.
- Argument by ad hominen; their making that mistake doesn't prove them wrong, though it is part of a pattern of their being idiots.
- Not putting up with this shit any more; because they're fascist shit-heads, that mock everything that is good in England every time they open their mouths.