I assumed this was just a stylistic feature until I started listening to some of the sample conversations on Japanesepod101, where the people speaking are all sober adults. Nevertheless, when some tasty noodles and a baby are presented at table, out come the "Oishii!!!" and the "Kawaii!!!!" once again. So maybe that is simply the correct way to pronounce these words? Or maybe Japanese people really are that excited about good food and cute things? Or feel culturally obliged to appear so?
It's hard to know how to gauge such matters. I was spurred to write this by receiving an email from the Boston Marriott just now, letting me know that they were "excited" at the prospect of my visit. Since I'm sure they're equally excited at the thought of all their other prospective guests I can't help but picture the staff in a permanent state of twitching frenzy, which isn't what I want in a hotel at all. However, I assume something got lost in the translation.