When designing one-off levels, it’s good advice to give the player the first Super Mushroom in an easy and safe environment. If you number your levels so a player knows there’s a progression, it’s understandable to make later levels more difficult. If the player likes your level design, even if a bit difficult, they will want to check your other levels, and will play through them in order.
A level is defined by its first Super Mushroom. In Super Mario Maker, the placement is even more important, because the player will not enter the level with whichever power-up they completed the prior level using.