My first thought is that it was some dude who simply wanted five minutes of fame and (sort of) got it.
My second thought was that it was an intentional security test, and someone wanted to see how far he'd get, anything could have been concealed in that stuff animal (or it could have been something else).
Let's say your a terrorist or a foreign leader and are considering an attack on The White House, and know you can get some suicide bomber to do it. The big question thus becomes how long it takes for The Secret Service to arrive, how long was this guy on the lawn?
By the same token it could have been an internal test to see how quick the SS responded.
See, you don't need to whack The President to make The White House a worthwhile target, simply the symbolism in damaging it can be a huge boon to a cause. Not to mention that a lot of very rich and powerful people have been known to rent rooms there (sometimes for the long term), I don't know who, if anyone, is currently in residence there, but those people could be worthwhile targets of opportunity.
Security is probably a lot lower when The President isn't there, so if this was a probe by an enemy, that might be what they wanted to test, to see if they could hit the place and do enough damage if the dude was say carrying a satchel charge or wearing a vest of C-4.
It should also be noted that we in the US have become too stupid and liberal in a lot of regards. Had The Secret Service gunned this guy down given how he was "obviously simply deranged" there would have been an outcry. As odd as this sounds, it also means that wearing a silly costume and acting weird can be a great way to get past security of any sort afraid of an incident. Say some dude wearing a Big Bird or Mascot costume packed with C-4 gets over the fence and runs for The White House (intending to blow himself up and simply destroy part of it for the symbolism), that might hamper SS reaction and get them to try and chase him rather than gunning him down or whatever. After all if they just shoot some whack job in a bird suit who jumps the fence the SJWs and media will be all over them.... and there is no way to tell... see where I'm going with this? One thing you have to consider is how much the Pokémon hat/plush slowed the response even if they pointed guns, and of course how seriously they took it, if he say had an Uzi hidden in that Plush and fired at The Secret Service, how many of them could he have taken down with a faux-surrender?
Of course understand in the current climate I'm exactly the kind of guy who would say there should be a sniper or three on the roof with .50 sniper rifles taking down any fence jumper on sight, no matter what they look like. Even if there is one on assignment there, obviously they aren't being that... assertive with their perimeter.
