That's a design question rather than a programming question, so moving to a more appropriate location.

Mario levels can be extremely difficult.  Go look at youtube videos on what to make (and what not to make) in Mario Maker for plenty of examples.

There are many things that establish "difficulty" in platformer games. 

There are levels where you must memorize a invisible objects, jump to a place that is off screen, or react with extreme precision.  Very few people like those types of difficulty.  A hard level of "search the entire level for a randomly-placed invisible block that holds the key" may be difficult, but probably isn't fun.

Some people enjoy a challenge where you approach and there are many moving parts to navigate.  Not "memorize this complex path" or "perfect twitch-reflexes required" kind of difficulty, but a series of actions that requires some degree of skill.  As a Mario example, two or three hammer bros working together are a good example of this.  Players can do it, but it requires a degree of skill.

Some people enjoy puzzle challenges.  Think less like Mario, more like Legend of Zelda. Don't do it with off-screen trickery, provide all the pieces in a visible form, and if possible provide multiple solutions.

Some people enjoy stealth challenges.  Other people hate them, so if they aren't central to the design they make a good alternate solution to puzzles.

Personally I prefer when there are a small number of steps involved, and it requires a mix of skill and thinking to overcome. Provide gameplay that is narrow but deep.  That is, provide a small number of elements that players must learn, and find interesting ways to combine them.  Some people don't like that, preferring a tremendous number of options, but it has wide acceptance.

As a Zelda example, provide a small number of tools and then explore many different ways to use them.  Start simple, slowly introduce combinations of tool use, then work up to patterns that require more skill but don't alienate the player's experiences.