I will put out this much of a disclaimer: the concept of "If ____ was ____" is so tired at this point that I'm generally unforgiving of it. It's a matter of plugging some concept into such an overused framework that the result is usually uninspired and manufactured, meaning that little thought was put into its conception.
However, I did appreciate "If Adult Cereals Had Mascots" for at least attempting (and succeeding at) doing something original with the formula. Honestly, if CollegeHumor had chosen a better title, I would be much happier, but hey, they have to get views, and by following the sh*tty industry standard, that's how they reel 'em in.
The premise is so simple that it's kind of baffling that it hasn't been done already. Basically, a guy eating oats for breakfast is repeatedly tormented by Revered Oats, a sad mascot for an equally sad food item, who fails in a series of unenthusiastic ploys to retrieve the breakfast. ("Woe unto me. Misfortune has withheld my precious oats yet again. Good day to you.") He's basically a sad version of the Trix Rabbit, as the video reminds us.
A lot of the humor is deprived from the enjoyable juxtaposition of the typically-optimistic cereal mascot contradicting one that seems to just be going through the motions, and it makes sense: that's adulthood, with the mystique and hopefulness replaced with a general sadness. Even when Reverend Oats gets what he wants, he doesn't seem to care, remarking he'll eat half now and half later, cold, because as the jingle goes, 'tis most prudent.