First let me explain to anyone who did not play Warcraft 3, or has simply forgotten, where exactly the Pandarian character made an appearance: in bonus/side/secret missions. Missions that were not part of the lore for that game. The space marine also has a model in WC3, is he part of WC lore? (Answer: Not yet!) At the time, the character was an inside joke brought to life by Blizzard. That they now claim that the Kung Fu Pandas are ‘serious’ is PR spin. That or the interns working on WoW today were too young to remember/play WC3.
Next, the Warcraft lore was the Warhammer IP. Feel free to Google up that whole tale, but its fact not opinion. Consider what WoW is today and you will see just how true to the lore Blizzard has been over the years.
Blizzard games have always included bits of humor. Everyone knows this. It’s why /dance in 2004 was as silly as it is today. But much like the Panda in WC3, /dance in 2004 was not really part of the game’s lore. It was a known side joke. Again, something that Blizzard has always done. Diablo has pretty serious lore, yet also contains such jokes. Would the Panda fit into Diablo 3? (answer: Not yet!)
All of that said, lore has never been a strength of WoW, in part because even in 2004 they had already strayed too far away from the source material. And really MoP is not about what jokes the Pandas will say or just how silly their /dance will be. We all know, today, without actually seeing it, that it will all be over-the-top childish and cute. We can all see the audience Blizzard is, and has been, shifting WoW to. That’s been covered. The panda is just the salesman pitching the gameplay, and the gameplay he is selling is not the same gameplay you bought in 2004, no matter how ‘serious’ you intend to play.
MoP clearly has gameplay aimed at children, and if child-like games are your thing, you should be excited for the expansion. If you were hoping for content and gameplay closer to what WoW once was, be that time period vanilla, BC, WotLK, or (god help you) Cata, you are not the audience Blizzard is looking for with WoW now.