I’ll be the first to say that if all you have is headphones, then by all means don’t let that stop you from mixing. Remember, it’s not what you use, but what you do with it that matters.
I disagree... up to some point this is all well and good. And it definitely sounds very beginner-friendly.
But the harsh truth is... that .. yeah, it's bullshit. Sound quality matters... a lot and in a lot of ways. Without great monitors you're working in the blind. A great headphone can be useful if your room and speakers aren't perfect (likely they never will be) but if you mix solely on them, and play it back on speakers it's gonna sound way off.
Oh sure, there definitely are cases of "yeah I literally mixed that down on my iPad while I was waiting at the airport."
Cool story, but those are always very knowledgable mixers with a decent amount of studio experience. They can rely on that knowledge and experience to compensate for what they can't hear on the headphones. Like for instance how sound acts in an actual space. It becomes very hard to judge dynamics, even before your ears get exhausted (much quicker on headphones.. you really have to take a break every 15 mins or so). There's a temptation to go wild with panning, mid/side, widening and whatnot because it all sounds great on headphones while sounding incoherent and floaty in a room (which is actually a cool sound to mess with if you're into making ambient!). So unless you have a lot of experience and intuitive knowledge of the gear and effects you're using, it's not very likely you'll make anything decent with just headphones. I definitely can't and I've been around a while.
Quality matters. Not just to the listeners. For you as the maker as well. If you want your music to be taken seriously, you should take it seriously.
Working with quality gear is more fun, more satisfying, gets better results more quickly and intuitively. It allows you to do more with less, while you're goalseeking your dynamics and mix, my excellent and large speakers get me 75% there before I even start thinking about mixing. Now what 'quality gear' entails for you is entirely personal. For one that's gonna be a laptop and field recorder, for the other a lab with modular synths. Whatever is exciting and fun for you to work with. Even then you don't want to skimp on sound quality... at all! If you're trying to take photographs, you're not gonna do it from behind a dirty window, right? Unless you're intentionally looking to achieve that effect.