Stale Move Negation

Is it a good feature?

It's definitely a flaw that should be removed. It makes comboing much easier when stale moves kicks in, and it's just annoying to deal with.

Brawl made it even worse

Brawl's Stale Move Negation is just horrible, making characters with limited KO potential to have even worse KO potential; e.g. Mario (his only finishers are his ground smash attacks, which severely limit his KO viability).

In care you're wondering why Brawl, it's because in the previous two games, knockback wasn't changed, but damage output was decreased. Brawl decreased both damage output and knockback. This allowed either characters to exploit it and make cheap combos, or it limited characters who rely on strong moves repeatedly.