Agreed, Jake Lloyd wasn't a very convincing actor, but saying things like 'Yippee!' is what little kids do. Besides, he lives on a planet furthest away from the bright centre of the galaxy, so style probably hasn't made its way there yet
And where was Anakin to get some more clothes from? Queen Amidala had lots of dresses, but Anakin would look kinda fruity in one of those. And Yoda said he could not be trained as a Jedi, so he wouldn't have dished out some cash for a little slave boy. Seriously, that's like asking why Jar Jar didn't dress up in a tux for the final celebration.