Nova the Squirrel is definitely above average for a platformer on the NES. Its a big title and in terms of the sheer amount of different enemies and items it's outstanding. But is Nova on par with the best of the best – I'm thinking here about Super Mario Bros. 3 and Konami titles late in the system's lifespan? Nah…
The controls are a bit too slippy. For example, when you are falling down touching a wall and there is a hole in the wall, pushing in that direction won't let go you through that hole and instead you will fall to your death (checkpoints are plenty though). I can't remember a single Mario (or come to think of, any other of the top crop of platformers) where you can fail like that. I suspect the game's programmer (basically the entire thing is one person plus another adding some music) put the physics first, asking:
Is the character moving at the right speed for x to happen? But for some things the most fun games are just directly taking the player's input to make the wish come true, plausible physics be damned. In Super Mario Bros. (and many platformers following that mold) you can just run over a gap the size of a block. I believe the check a Mario game does is all about whether the player touches forward plus run button or not, the check is not about the actual speed of the controlled character. In Nova, you can also run over a block-size gap, but just by normal walking,
and running lets you cross a two-block gap. Why is that? My hypothesis: The programmer wanted the block-gap trick like in Mario, thought about it as a question of physics above all else, and so added a bit of safety to the physics check for that.
The character is lacking somewhat in variety of animation: walking, running,
and even swimming all look the same. Based on the projectile you have, pushing up or down may change the direction of it, but your character doesn't change animation for that. Ideally, carrying the item should be visualized on the character and not just the HUD, and the pose when pressing up or down would tell the player whether the weapon is the type that can be aimed. Likewise, enemies could benefit from more animations, especially telegraphing actions a bit in advance. (The Rayman games do this a lot – the spikes, the flames, and so on – IMHO they do that a bit too much. I bet Ubisoft has explicit design guidelines for that.) Some things in Nova have literal countdowns for attacks displayed, but that's a bit too sterile compared to animation that lets the player anticipate action. There is also no indicator for when enemies are about to get out of stun (they should vibrate a bit right before that or open their eyes or something). There are some other little things missing that make the difference between good and platformer perfection, e.g. you have a boomerang that lets you grab things, but you can't affect its movement by moving around yourself.
The engine of the game has many features: There are moving platforms, trampolines, locks and keys, blocks you can shove around, switches that turn blocks on and off, mine carts you can ride in, and so on. You could take just half of the stuff and make a game with it and it wouldn't feel like a sparse affair. The game isn't just free as in free beer, the source code is published under GPL. This could grow into something like Mario Maker, with results that can be played inside NES emulation and on rewritable cartridges that you can put into a literal NES to play.
https://novasquirrel.itch.io/nova-the-squirrel>While the tools are not very user-friendly yet, if you feel like getting into the code you can build new levels for the game with a drag-and-drop level editor, or modify the game as you please.