Learn English – What does “down with homework” mean

Green's Dictionary of Slang lists numerous meanings for smash, but the one that appears relevant is:-

smash n. a great success, a 'smash' hit

from which you can get the adjectives smashing or (if you like) smashtastic.

Other meanings are possible, but if your context suggests a compliment, that's the only one that could apply.

See also here, meaning (6):-

Informal A resounding success: The play was a smash on Broadway

Edit: per your edit, certainly. One could be a smash on the stage, at a party, at some sporting event or just generally.

As BlessedGeek has noted, OshKosh B'Gosh is an American brand name of children's apparel, as well as its main slogan/tagline. Oshkosh is the city in Wisconsin where the firm was founded. The town in turn was named after a chief of the Mamaceqtaw (Menominee) people, in whose language the name means claw.

B'gosh is an abbreviated way of saying by gosh, a bowdlerization of by God.

Without the context, it is difficult to say what was intended by the reference. The character might have been evoking Midwestern wholesomeness or blue collar patriotism, or might have been mocking something or someone as corny, or might simply have been making a reference to the company or to Wisconsin.