"History Lesson" by Arthur C. Clarke, first published in Startling Stories, May 1949, available at the Internet Archive. The alien scientists are from Venus. Here is a plot summary from Wikipedia:
The first part of the story is told from the perspective of a tribe of nomadic humans in a future where Earth has entered a final ice age. The tribe is travelling toward the equator ahead of glaciers that are descending from the North Pole, but discovers, when they arrive in the last hospitable region of the planet, that glaciers from the South Pole have already almost reached them. The tribe carries with it a few relics from the mid-21st century which it considers sacred, although the functions of the various objects have been forgotten. Before the ultimate extinction of the human species, the relics are safely relocated to a mountain that stands between the two advancing bodies of ice.
The second part of the story is told from the perspective of a race of Venusian reptiles who have evolved into intelligent beings capable of space travel in the 5,000 years since the cooling of the Sun. The Venusians travel to Earth and recover the relics of the last tribe of humans, now the only remnants of civilization not buried under ice. The title of the story comes from the attempts of the Venusian scientist, to reconstruct the life and times of erect bipeds that once walked on the Third Planet through the analysis of one of the last relics of mankind — a film reel that apparently contains a Disney animated cartoon short, which ends with a section of text, which no amount of effort and speculation can decipher : "A Walt Disney Production".
The Disney cartoon character is not named in the story, but from the description it's obviously Donald Duck, not Mickey Mouse:
Once more the final picture flashed on the screen, motionless this time, for the projector had been stopped. With something like awe, the scientists gazed at the still figure from the past, while in turn the little biped stared back at then with its characteristic expression of arrogant bad temper. [Emphasis added.]
[P.S. But the uncredited artist who illustrated the story for Startling Stories agrees with your interpretation, as Mickey Mouse is shown in the illustration.]
The punch line:
Its secret would be safe as long as the universe endured, for no one now would ever read the lost language of Earth. Millions of times in the ages to come those last few words would flash across the screen, and none could ever guess their meaning:
A Walt Disney Production.