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Well, "motivational" usually refers to the stuff that encourage or give psychological support to the viewer, while "mouthpiece" usually refers to stuff that encourages you to weld the doors shut and burn the house down of someone who said a bad word on Twitter 13 years ago.
Essentially, "mouthpiece" refers to stuff that uses a character from the show to parrot an opinion the artist who drew it has, regardless of whether or not the character would have that opinion in canon or not.