A lot of folks advocate minimal DM prep in favor of improvising during play. This approach definitely has benefits, not least of which that it encourages more folks to run games even if they have otherwise busy lives!
But I also advocate for more substantial GM prep as long as you do it in a “healthy” way. What do I mean by that?
You have time for it. Don’t try to spend more time on RPGs if it will cut into things in your life like work or school or family or a relationship (e.g. with a partner or close friend). Gaming and downtime should fill that part of your life that helps you recharge, but never to the extent that it negatively affects more important things.
You enjoy it. If you don’t like spending time on prep, don’t do it! Find some other way to spend your downtime, and enjoy your game your way! But maybe you’re like me. Maybe the idea of cooking up new monsters or new factions or new maps or whatever really gets your juices flowing and feels like “play” for you. Then go for it! I spend a lot of my prep time perusing images on Pinterest or getting in the heads of major NPCs to figure out what they want and how they plan to get it now that the PCs have interfered with their plans. It doesn’t feel like drudgery: it’s enjoyable.
You can reuse what you prepare. Inevitably, your players won’t go to the dungeon you drew up, or won’t talk to the NPC you created, or won’t find the magic item you designed. That’s okay; file it away and find a reasonable place for it again in the future. Note that you should not just make their choices immaterial and send them to that dungeon anyway! But if you have a really interesting set of encounters in a temple of an evil god, maybe later they’ll want to come back to it anyway. Or maybe later you’ll need another temple, and you can tweak this one and use it then.
Never feel bad about enjoying RPGs your way. If you find yourself enjoying your preparation time and you’re doing it in a way that strengthens, not weakens, your sessions, then keep going!