Nintendo clearly has no right to go after other platforming games that have copied from Mario’s mechanics, so why should they have the right to go after games that use Mario’s aesthetic or name?

This argument doesn’t hold water to me. How are you conflating a brand with all games in a genre? IP and brands are a signifier of what to expect. Sherlock Holmes is a great example of what used to be an IP known for a very specific style of mystery story but now just means “genius problem solver maybe with a drug habit.”

Fan games are not normally morally wrong, but I do think it’s kind of trashy to try and make money off of someone else’s brand if you’re not doing it out of passion. I just don’t see it as a legal problem

We are going to just have to agree to disagree. Spiritual successors happen all the time. The reason people make fan games is that there is a lexicon built into their project already. It’s a shortcut instead of building (and considering) what is meaningful to your game in a lexicon. Additionally, a lot of people do not consider how their changes affect the lexicon throughout all games. So what you are left with is mostly people who don’t truly understand, never talked to the creators, never worked with them, assume everything from a product perspective, pushing out something that adds to the brand without any true coherency or consideration of future titles.

I for one see it as wrong to attempt to take someone’s work and not only pass it off as your own but also potentially break their ability to make future iterations of their work.

Look at Sonic P-06, a fan-made remake of Sonic ‘06. It’s highly polished, has fleshed out features that never made it into the original game, and is absolutely a labor of love.

Perhaps a labor of love. From my initial parsing of reviews, looks like they didn’t attempt to change anything but kept close to the source material.

Most companies’ strict protection of their IP just prevent works of art like P-06 from seeing the light of day

I feel like that’s okay. There are plenty of original and better ideas out there. It doesn’t prevent things from being made. P-06 being “still terrible but better” doesn’t really say anything. The fact that games like Black Mesa, Sonic Media, Skywind, and very specifically Zera: Myths Awaken also exists, a game that started as Spyro 4 and Activision sent a C&D, so they rebranded. These things are easily fixed and honestly, it’s great if fans want to attempt to build a game off of someone’s IP then ask the IP holder if they can continue with it. If not, they can just rebrand and create their own universe. Temtem is a great example of what happens when people are forced to create their own thing. It becomes more impactful and allows for a more interesting product. Not just a cookie cutter game.