There’s a difference between that. Fluttershy probably thought Rainbow could take it because she usually likes to come off tougher then she is inside. But Pinkie should of known better, back in Griffon the Brush-Off she wouldn’t even do a harmless prank on Fluttershy. Her behavior in Filli Vanilli seemed worse then just a prank.
“But Rainbow Dash, just because you failed the Sonic Rainboom a hundred-thousand times in practice, doesn’t mean you won’t be able to do it in front of an entire stadium full of impatient, super-critical sports fan ponies.” (Fluttershy to Rainbow Dash in Sonic Rainboom)
“Yeah! It’ll be so amazing! You’ll be there on stage, basking in the hooflights! The center of attention! A shining star! Everypony staring at you, judging you, jealously noting how they could be way better than you? Why wasn’t it them? Why wasn’t it them? And then, when you choke, they’ll turn on you, becoming a seething, angry mob, and you’ll be horribly humiliated, never able to show your face in Ponyville again!" (Pinkie Pie to Fluttershy in Filli Vanilli)
And I don’t know about you, but at least Fluttershy’s line sounds like it’s trying to be supportive even when it’s really not. Pinkie’s is just pushing Fluttershy’s stage fright fears right in her face. It’s all about intentions. Fluttershy thought she was helping Rainbow Dash, what was the intention of Pinkie’s line? To scare Fluttershy even further?