I, for one, don't think the sadness "shown" should be so obvious. Sometimes, the people who are always cheerful and happy go lucky might be the exact opposite inside. They choose not to show cause they are afraid that they will be judged, left alone and/or laughed at, thus, they prefer to show another face, something everybody else would like (smile, laugh, joke etc) so they won't feel completely alone.
Let's take U;Nee (Korean singer) for example. She always seemed cheerful through her career (lives, making offs, interviews etc) but in the end, she killed herself, just before the release of her 3rd album (and only few songs are sad, the rest are dance/R&B). Only 2 or 3 times she showed her true sadness but most of the time she hid it behind her smiley face.
And about the happy partying thing... a lot of celebrities (Amy Winehouse, Whitney Houston and more examples) try to make themselves feel better with drugs, clubbing, partying, alcohol etc. cause they cannot hold the pressure of being famous any longer, with all the paparazzi trying to disturb you every day and then show your awkward poses to the entire world, ready to talk trash about you. And they were smiling on the outside for a moment when partying and that's it, they go back on their daily depressed mood, feeling locked, no freedom etc.
The same thing might represent the Party Queen covers, even if she looks that smiley, she still is unhappy inside (hence she looks like she's high on drugs/alcohol, stripped to her panties and bra) and Ayu said that the covers are a parody of the scandal magazines, maybe that's why the photos look so unprofessional, they wanted to appear like a paparazzo took them and show how the "Queen" (Empress of J-pop) is so worn out and loose in the party.
Just my interpretation though
And sorry for the rant, I was kind of tired to see posts like "SHE'S SAD??? BUT SHE LOOKS SO HAPPEE DER!!1"