Delta’s champagne - usually Lanson or Charles
Heidsieck - is pretty credible stuff, and the price per bottle and quality is competitive with other airlines in J.

The catch is that the predeparture beverage is poured as cheap cava or Prosecco. If you’re judging the champagne based on that glass, that’s why you’re unhappy. Best to skip the predeparture beverage and instead order the champagne off the cart once aloft.