The short answer is yes. We haven't had a Ramanujan in decades, and they were more or less always extraordinary. Insofar as you can generalize all of its subfields together, it's reasonable to say mathematics is an extremely mature discipline now.

What I mean by that is there isn't much low hanging fruit around. Even when Ramanujan was dazzling Hardy with his results, many of them were already known. That was over a century ago. There just isn't much fertile ground left where a single brilliant mind can make significant headway based on raw talent or intuition, without first working through significant education in prerequisites. It's more and more commonplace for authors to collaborate on their work, because compelling problems at the forefront of mathematics research are increasingly requiring cross-disciplinary knowledge to resolve.

To focus on Ramanujan a bit in particular - one of the reasons Ramanujan was so talented was because he not only came up with novel results in complete isolation; rather, he also came up with original perspectives and theorems. That means he was capable of posing interesting questions, which is usually much more interesting and inspiring for further research. That's very inspiring, but the reason I point it out is because Ramanujan wasn't solving open problems in the West left and right so much as he was exercising ingenuity to pose - and resolve - open problems in fertile ground he could reach. But a century later, the amount of existing mathematics has increased so much in both breadth and depth that a burst of creativity is almost certainly not going to get an undergrad to find something that e.g. Rudin hasn't written down somewhere already. But more importantly, it's vanishingly unlikely they will resolve a famous open problem which is the subject of intense research interest. All the stories of untrained amateurs doing that are from the mid 20th century or earlier.

To turn your question on its head a bit: consider how astonishingly brilliant an untrained individual would have to be find a nontrivial result that the rest of the mathematical community hasn't found. Either they're inventing their own definitions and discovering entirely new mathematics in isolation (wow!), or they've somehow found a way to use the tools accessible to them (university analysis and algebra, at best) in a way every other trained mathematician has not. It's rare even for senior math undergrads to publish nontrivial research on their own. It's significantly rarer for that research to improve progress towards an open problem. The only example I can think of off the top of my head where undergrads actually resolved a well known open problem is AKS, and even then it wasn't a solo author.