Who was the first cartoon character to appear on the postage stamp?

That would depend a great deal on how you define a 'cartoon character.'

In 1995 the US issued Scott #3000, a sheet of 20 stamps commemorating a dozen of the early American Cartoons, some of which are still running in newspapers today.

The first cartoon character is arguably the one issued in 1984 that pictured McGruff, the crime dog. Bugs Bunny was the first animated cartoon character to appear on a US postage stamp in 1997.