@Background Pony #843D  
Because she knows that farming is silly, and to be called out on it makes her upset.

Why does applejack not look happy?

@FanOfMostEverything  
lol

@Gustauve  
“On second thought, let us not go to Sweet Apple Acres. It is a silly place.”

As a farmer’s child, I can honestly attest to the fact that farm folk are, indeed, silly.
 
They repeat the same labor intensive process every season, gambling their toils on the fickle weather, hoping that ‘This year will be better.’
 
They also enjoy the aspect of free child labor a little too much, and have many children in order to get the most work for the least (nonexistent) pay.
 
My parents had 16 children, and none of us are farmers. Do you know why? Because farmers are silly folk and we decided that we didn’t want to be silly.
 
So, yes, Pinkie is absolutely correct in stating that Big Macintosh is, in fact, a very silly pony.