Are there any stories were an MLP character that wears a diaper that inflates like a balloon

No idea

https://www.fimfiction.net/story/481393/lunas-inflatable-fun

I think some of that happens in some of mine, though not as the main focus so much as a thing that happens a lot. Quite a lot of the Bagpiperix stories have diapers puffing up for example.

7441137
can you send me a link to his story

7441180
This is the first of the Bagpiperix stories:

https://www.fimfiction.net/story/430526/bagpiperix-in-ponyville

Please explain it to me why is inflatable diapers good. Is there a logical purpose behind it?

7441182
I suspect that there's an element of "big diapers", plus "inflation" (which is popular) and the comedic silliness too. It's a kink, it doesn't have to 100% make sense.

7441200
Big diapers makes sense to be. But inflatiation ... I just cant wrap my mind around it.

7441217
That would be because it's very big.

On a slightly more serious note, I think the thing with inflation is that it's always seemed to me to be fundamentally silly. And that's part of the point of it.