One good disguise the Trix Rabbit once used was to somehow turn himself into a cute, adorable bunny (they used a real rabbit in the commercial) and changed tactics, not talking at all and simply trying to sneak up on the kids. Unfortunately, his addiction to it gave him away as it always did.
Similarly, Barney Rubble in the Pebbles commercials, who's been doing this since 1978.
This 1972 commercial was one of the first to show Barney pulling this stunt. In this case the ONLY change was putting a fake beard on. And he still managed to fool Fred.
An ad for cheap cell phone service featured a woman who was saving money for her expensive cell plan by having her son travel free on a plane trip. She'd dressed him in a floppy-eared Halloween costume and stuffed him inside a pet carrier; hearing them converse, a baggage handler marvels at the "talking dog".
The dog from the Bush's Baked Beans commercials recently appeared in a labcoat and false mustache, posing as a food science researcher. For those who haven't seen these ads, be aware that the dog is a real golden retriever, not a cartoon mascot.
He also shows up as a Bedsheet Ghost of the recipe owner's grandfather, but is revealed when the man explains, "Grandpa didn't have a tail."
Seen in a TV ad for Speedway, featuring a contest for their Speedy Rewards card loyalty card. A man keeps coming in and using his card, wearing a series of ridiculous disguises. Finally, the clerk, who isn't fooled for a minute, tells the customer that he can use the same card as many times as he wants and still be entered in the contest each time. MST3K Mantra, since if you think about it for more than a few seconds, you realize that no matter how well he disguises himself, the card always carries the same computerized details about his identification. (Then again, he might not be smart enough to realize that.)
A Dunkin' Donuts ad had the company donut maker (a portly little guy with a moustache) spying at a rival donut place dressed as a woman, coyly holding a hand over his moustache and not fooling the anxious counter man.
In one McDonald's commercial from the '70s, Ronald dresses up as a mailman to thwart an attempt by Grimace (who was evil at this time) to steal some milkshakes. The disguise is little more than a hat.
One of the Naked Gun-type ads for Red Rock Cider has the Comedic Hero working undercover in an ice cream van called "Mr Policey" and labelled UNMARKED POLICE CAR. This naturally leads to a Visual Pun where a customer asks for a couple of cones and gets two traffic cones instead.
Frank Drebin: "A hot tip led Sgt. Doughray and me to Ascot. We mingled unobtrusively for a while before dividing forces." (Doughray and Drebin are wearing dark men's suits and fancy ladies' hats.)