Merch by Amazon/AMoD is insanely saturated. They currently reject lots of people who apply, and as far as anyone can tell, it has nothing to do with the person, but seems to be the way they keep growth in balance with their capacity. When you get accepted, you get to list a whole 10 designs--and no more, until your sell 10 items. Then you get bumped all the way up to 25. I'm at Tier 10,000, but actually only have just short of 500 designs up. I started in early 2018, but haven't done much with it since late 2019 when the listing Rejection Bot got ever more hyperactive (and usually wrong) in its actions.
Which is not to say that MBA/POD is not an opportunity. It is.
But what do you have that you can bring to the table? Is there some not-already-exploited hobby/occupation/niche that you have a passion for and a lot of insider knowledge? Tossing up the 10-Billionth dabbing cat design isn't going to get you anywhere. BSR matters. Listings without sales get their random shots at garnering sales, but soon sink to way, way, way, below the threshold for ever being shown in search results--unless they manage to score sales before then. Search for something like "Halloween Shirt" and you'll get 4 or 7 or however many pages Amazon currently sets as the truncation point. But there are a million (or more) Halloween listings if there is one, most of them simply will never be seen again if they don't get those initial sales. You can chase after those initial sales by paying for ads, Jeff does need more money for his rocket 🚀
But perhaps you were thinking of doing like an Etsy shop with Printful or Printify or Custom Cat handling the fulfillment? It costs money to list on Etsy, so best to tightly focus on a niche and really stay on brand. As opposed to the POD marketplaces where listing is free, so one can throw endless "poop" at the wall just to see what sticks and what doesn't.
But to your question. Could someone start now and make money? Absolutely. Could someone start in 2018 and give up in frustration? Yes, many did. It is harder now than then, but it has always depended more on the person than the times.
If you have some sort of passion about a niche or art or shirts or whatever, go for it. There is no way to know other than to try. If your only interest is chasing $, you'd probably get more for less investment of time elsewhere.
