Nintendo Switch

Out of curiosity, do you mean SMB2 USA (one of my very favorite games of all time!) or SMB2 JPN (rage inducing difficulty on purpose UGH)?

In regards to the topic, I don't think it's the difficulty that's bad in old games, but the outdated game design philosophies. Get rid of extra lives and just let the player keep trying. Don't punish the player by sending them so far back, making them replay what they've already played a ton of times. No unfair deaths, losing should ALWAYS feel like it was the player's fault for making a mistake. Don't make the player wait very long before getting to try again. Give the player options to skip a difficult section (for now) and play other stages. Provide powerups they can earn to get past difficult sections.