Mub and Grub are clearly in the movie just for yucks. Much of their visual humor involves all the fun things they can do with their extended eyes, like when Mub mocks Nod with how his eyes are sunk into his head and therefore cannot see in all directions, and Mub retracts his own eyes into his own head to make for a very hilarious and disturbing image.
When Nim explains that everything in the forest is recorded in scrolls underneath his tree down to the very minute, Mub and Grub keep asking, "can they record this??", and perform some silly stunt. Once Nim, weirded out, gets the scroll that just recorded all their antics..... he turns and hastily rips it apart.
Ozzie is also in the movie for largely the same reason, because whenever he runs he hilariously knocks over several objects that do not seem to slow him down.
Any scene with MK trying to adapt to being small. The scene with the mouse is hilarious!
The mouse is, at first, very cute, but because MK is a centimeter tall, it seems like a gigantic, hulking, blood-thirsty monster! ... then it reverts back to being cute after Ronin scares it away.
The bit where she clumsily makes it into her dad's place with Nod and Ronin is also great.
Professor Bomba screaming "I HURT MY ELBOW!!" in slow motion voice while chasing Nod, Ronin and MK in his office.
The Leafmen call humans "stompers", and Nod and Ronin relish in making fun of the slow, lumbering, clumsy oaf in front of them. Not realizing said "stomper" is the father of MK. Who is standing right there. Awkward.
Even better is that when they're mocking him they say the "I HURT MY ELBOW!!" line, making the above example a Brick Joke
Gets especially funny when MK angrily points out that Ronin knew she was a stomper.
Ronin, of all people, goofing off with Nod using Bomba's animal skeletons.
The two Boggans fighting when Dagda walks past them.