Many games, not just the original SMB2.

Rock Man 2 (Mega Man 2 in North America) had one only difficulty in the Japanese release. In North America, Mega Man 2 offered a "Difficult" option, which was the original Japanese difficulty. "Normal" was the default difficulty for the North America release toned things down quite a bit, as Capcom felt the game would be considered too difficult by the North American market.

There's well documented lists of games that were significantly easier for the North American release, with only a few harder for North America.

In fact, the difficulty tuning was usually harder within the first release of the game(regardless of location). So games that saw a US release first were hard there, while games released in Japan first were hard in that version. And since the majority of the NES library derives from Japan-first releases you get the "Japan version is harder" effect.

There are some exceptions to the rule of course, but it usually holds that the later localizations get the more polished product.

Games adjusted to be harder for the US can have really punishing levels of difficulty though. Like the US Hard Corps release is excessively hard.

A counterexample: Resident Evil was actually harder in the US because they removed the auto aim feature, specifically so US game renters would have a hard time beating it. I'm sure there are more examples of both kinds of modifications for the US market.

You're not in any danger of finishing Final Fantasy in a few hours. The Lost Levels is not really a very good game.