@Napsack  
I just checked the sprinkler scene in Epic Fails and her hair keeps its style there, so this could be a case of both then.
 
(Also, given her usual personality, she seems a LOT more miserable about being rained on (or whatever has her feeling down) than Sunset, Sci-Twi, or Fluttershy. Or Applejack if you think she was in the rain.)

@Background Pony #1949  
No, her hair is definitely wet in both. See the shiny streaks in her hair? That’s the sheen of wet hair in this show, pretty much.
 
I would say that here is pictured a difference in attitude to being wet. On the left she’s just had rain dumped on her and she’s cold, uncomfortable, and just a little bit miserable as a result. On the right she’s at the beach with her friends presumably having fun swimming with them, and is generally having a good time.
 
I don’t know if it’s an intended difference or not, but it is pretty interesting regardless. Pinkie is a very expressive girl.

So the left image IS just her hair being deflated. (I thought so based on her hair re-inflating when she cheers up)
 
Makes you wonder, how was she NOT in the rain? Seeing as how she must have arrived just before the rain stopped, yet apparently her hair is dry (Rarity’s hair is messed up from drying it and Fluttershy must’ve been in the sun for a little bit after the rain and looks like she’s been through more than just rain in any event, and Rainbow Dash probably started running 10 seconds after the rain stopped, at the speed she was going at, not sure about Applejack but she’d also be in the sun for longer). Did she put on an umbrella hat ahead of time? (I get the feeling a cheerful Pinkie would put an umbrella hat on, where a gloomy Pinkie would walk in the rain) Or is it just a Pinkie thing?

@immblueversion  
we know that you oxymoron [we were talking about the right one being the wet hair]

The left image isn’t supposed to be wet hair. It’s supposed to be Pinkamena hair. Now think: what the hell happened that could’ve made Pinkie go Pinkamena?

Funny, Pinkie’s personality is like 40% hairstyle. She looks like a background character now.

The wet look kinda suits her.

Hmm… Think about it people.

Pinkie se ve bonita con el pelo lacio. :o

Pony Pinkie in the human world so I can imagine (right side)

The implication in Monday Blues is that Pinkie’s in a sour mood because it’s Monday. Her hair automatically poofs out for the group photo because she’s happy to be with her friends again. It’s the same as when she straightened it in the first movie.
 
Contrast wet-hair Pinkie actually having wet her poofy hair and we get a different look.

There a difference between a dipped hair and a showered hair dipped hair retain it’s general shape while showered follow the waterflow

Wet Pinkie’s new way of drawing her hair makes a bit easier to adapt straight-maned Pinkie to the human world.

They’re making it to look her pony counterpart