They never seem to be able to “grasp” the concept of how to properly hold a flute.
I used to be bothered by that and then I tried to draw a flute player. IMO, it’s really hard to get across … The viewer can’t tell that the stumplike thing behind the flute is actually someone’s right forearm and that the little blobs on the RH keys are fingers.
The violinist’s bow hand position isn’t going to win him any prizes, either. 
I thought about that when I first saw it and I agree that it’s a tough one to draw. It’s funny though, he looks like he’s about to start mackin’ away on a cob of corn. 
Out of interest, just what is a “classical instrument”?
Anything that’s not a football.
OK. That completely clarifies the position for me.
If cartoonists would think about grabbing a vertical stick then moving it to the horizontal, they’d get it right. Instead, eating an ear of corn or slice of watermelon seems to be the starting point.
A “stumplike thing”, and “little blobs”. Of course they can’t tell what it is. The prosecution is done with this witness, your honor. 
!
True, there is that.
They don’t all get it too wrong:
I guess he’s playing ‘Row, row, row your boat …’
Ha! I forgot to include the caption: "It’s definitely a change of tactics, but the over-all strategy remains the same.”
Now we can tackle the stance. I don’t play looking like I have to pee. 
wow! how cool is that?
Yes, but otherwise, is it a good likeness? 
I wonder if you heard me splutter over the pixels…
Well, look. For one thing yer man can’t stomp his foot. All he can do is look like a coy maiden mincing in a grove or something. Not that there’s anything wrong with that, all you coy maidens in groves out there, but c’mon. As a guy fluter I decry the stereotype. 
Fie on your decrying, sirrah! Fie, I say! And thrice fie!
[I have no reason for saying that, other than it sort of rhymes, and I kind of like the word “fie”.]
I think the guy in that last cartoon is playing this:
I’m sure the experienced galley slaves here will find something. Like their hands are too far apart for leverage, or long beards get caught in the oars …
Hey, no hiding secret messages!