A significant part of the community shares a similar sentiment. The official JJ2 levels and enemies appear as if they hadn't been designed with capabilities of the playable characters in mind. It is presumed that, during development, players were gradually given more power - such as the ability to see more thanks to increased resolution, the ability to constantly run at full speed, and special moves - and the obstacles they face weren't updated to account for that. As a result, sufficiently skilled players can feel invincible in the official campaign and since it's hard for them to find a challenge, it's easy to lose interest.

In spite of that, I don't think I agree with your reasoning. It's only natural that some of the enemies and locations from the first iteration of the game would be transferred. It provides a sense of continuity. You wouldn't complain about Mario fighting Goombas in the Mushroom Kingdom again - that's just where he lives and who his archenemy's henchmen are. JJ2 does not even revisit that many themes of JJ1 outside of Flashback, the episode specifically meant to be a throwback to the original game. It makes sufficient sense given JJ2's time-travel-based storyline. And despite borrowing old themes, the game goes to great extents to refresh their looks and give them new mechanics.

JJ2 still introduces new themes every two levels: castle, Carrotus, laboratory, colony, Alice in Wonderland, beach, jungle, and two different iterations of hell, and the palettes alternate between day and night on each level. I find it hard to see what your complaint about visual variety pertains to. Gameplay variety, now that there's not a lot of. Many levels introduce new elements, such as the flying carrot in Castle, the airboard in Labrat, pinball bumpers in Tubelectric or falling rocks in Jungle, but they seem to fail to follow up on them and turn them into a significant recurring theme. A commonly used rule of good design is: introduce a new element in a safe scenario, then reuse it in a dangerous scenario, and then start mixing it with previous elements. JJ2 doesn't do that. Often it just sort of introduces a new element and then abandons it forever. Its levels do not show a lot of thought put into them.