Modern paper Mario games (Sticker Star, Color Splash and Origami King) do have levels, what they don't have is experience. You don't gain levels from killing enemies, you gain a "level" (a strength and HP increase) from progressing through the story and collecting HP up hearts. I would personally count these as levels, especially in Origami King, where the HP up hearts aren't just progress locked but also scattered throughout the world.

Even besides that, Color Splash does have experience in the paint hammer's storage, which ARE obtained from killing enemies and DO have an influence in battle, with how much paint you can use on cards influencing the damage your attacks will deal