Grizz, Panda and Ice Bear, for instance, when trying to keep Charlie a secret. George of the Jungle. Pearl from Steven Universe. The musical actors from Avatar: The Last Airbender. Anyone with a secret identity: Ben 10, Hiro from Big Hero 6, Juniper Lee, Martin Mystery and Diana etc.
Usually they still get away with lying, even if it's just for a short time. The audience notices it's a lie, but the characters don't.
Now I think of it, Andy from What's with Andy? is really good at lying, but he's a prankster.
Not a cartoon, but Harry Potter is an excellent liar.
Also the kids from Stranger Things. Kids in real life lie like crap all the time, and never shut it off, so the good kid-based franchises are able to make a lying kid sound sincere to an outside observer. That probably extends to animation too.