I don't think they are flimsy at all. Until now, with the sole exception of Pinkie Sense, we have seen no pony give Pinkie's antics the kind of reaction you would expect if what we see is also what her friends see. Frankly if they saw what we see, Twilight should be a nervous wreck in a mental institution by now, because absolutely nothing makes sense. Instead we see her surprised at Pinkie's sudden appearances (which is not an uncommon experience in the real world either, ever had someone sneak up on you?) and generally thinking she's being silly and hyperactive. Nothing unusual at all in their reactions to her (again aside from Pinkie Sense). You can't even explain this kind of total nonchalance from her friends by suggesting they've simply given up.
Sure, some of it might have a hint of truth to it, but for the most part, by far, I think what Pinkie does is vastly exaggerated for the sake of comedy, because it's a cartoon, and that's great, you know ... cartoon physics go well with cartoons. My problem is that if you wanted to use the setting seriously (whether in the show or otherwise), you have to discard the cartoon physics, and that's harder if the cartoon physics are suddenly complete canon and not just visual gags.
And I get that feeling that the writers generally follow this line of reasoning too, even if they probably aren't hugely conscious about it. I think they have a sense that having the characters acknowledge Pinkie's cartoon physics is probably not a great idea in the long run, and this one just slipped and wasn't big enough for anyone to think about it much.