Wario's Castle in SML2 is definitely a jarring change in difficulty, I could never beat it as a kid. Nowadays I can beat it, but it is definitely a chore.
As for the shooter sections in SML1, I knew a lot of people growing up (including me) who found those sections very difficult. I know now to watch oncoming bullets and avoid them while just shooting. Tatanga (final boss) is definitely tough compared to the rest of the game, the only strategy that works is go up, down, up, down, up, down... So that you can easily avoid the next bullet.
Come to think of it, Kirby's Dream Land 1 also had a shooting game level, the boss fight against the blimp cannon (can't remember its name). I don't remember another Kirby game with a level like that.