You did a quite good job on this one. When i look at it, i can basically feel the emotion Pinkie has. I also like how you made up the clouds. They look pretty realistic. And that's where we come to the first thing i'd like to mention. I think that Pinkie should have no outlines here. It makes her look too show-accurate for the background and it also creates the impression of her being closer to the viewer as the balloons are. Speaking of the balloons, the yellow one doesn't seem to really have a weight to bring up in the air. The string seems too slack for that. Another thing is the wind in front of the balloons (i guess it's wind, because clouds wouldn't make sense there). It also makes them seem to be more in the background. And if it really is wind, i'm curious about Pinkies hair which doesn't look like she'd be moving fast enough to create those wind-lines (i don't know the technical term for that x3).
All in all you made one pretty good picture here that really gets Pinkies feeling across. I guess she's even so happy that she forgot her cutiemark... Ok, that was lame. Ahh however. Good work