It depends on what you are looking for. Of course you want money, but if this is a side project, how much time can you put in?

Time dictates results, not the other way around. If you want something that's cheap on time, mobile is not the way to go. Even the most basic "Hello World" app still takes at least several hours to get past the App Store publishing step (figuring out a good description, keywords, screenshots, icons, etc...) And even then, you'll still probably need to make a website for it so you can point users to your privacy policy and online support.

Websites are far easier. All you have to do is nab a domain and upload some HTML to a provider. I can make a website from scratch and have it running on Netlify in probably ten minutes. Backend is where everything gets complicated, but depending on your concept, it may not take that long. Not to mention that your app most likely also needs a backend so, might as well bite the bullet. I made https://bongocat.io in a single weekend while procrastinating studying for midterms. I don't think I could have gotten a better turnaround even if I used the magic cross-platform mobile alternatives (Flutter, React Native). I've used both at this point and while they are a dozen times better than the native Swift/Java experience, at the end of the day it still just takes time to take the idea from your head and put it on a screen.

But if you want money, it's clear mobile is where it's at. Especially gaming. The only sustainable way to make money off a website is to build some kind of SaaS platform or...sell ads. It's really, really hard to build a user base large enough to the point where you can begin to sustain yourself off of subscriptions or single time purchases. You almost always have to offer some other software that piggybacks off the website (think game clients, business tools, etc) for users to justify spending money. On the other hand, IAPs are a way of life in the App Store, and monthly subscriptions are gaining traction ever so quickly. I can lock in to a $59.99/mo subscription in under one minute from an iOS app - if I had to do that on a website, I'd have to trust my credit card information with the site owner and I most likely would not do that. Even if I did trust them, I'd still need to put in my card, and that's a big hassle too.

So, if you want an easy side hustle where your goal is to get attention first and then maybe profit later: site

If you want big money and are willing to put in the effort for a few months before anything comes from it: mobile

If you want a serious business: you have to do both. There's no way around it, every mobile-first business needs a site to have a presence to people who don't have the app yet. Otherwise you let your entire business's brand rely on App Store algorithms to show it high up in rankings.

If you think you're good enough for social media: I mean, just do it already. The effort you have to put in to think of a good Tweet is minuscule compared to publishing an app. Podcasts, Instagram photography pages, funny Twitter accounts - it's never been easier to put yourself out there and get positive feedback. You definitely need an audience before you can even think of making more than a few cents off your fame, but the money is there, and if you just stay consistent and post often, you can probably make it. But that's the important part: stay consistent and post often. I'm of the opinion that the content you post doesn't really matter, as long as it engages your followers. So don't worry about being perfect from the get go. Just start.