In other words, "the universe is a simulation" is an unevidenced assertion, much like the multiverse. Yes, there may be some extrapolations of the underlying math that might point in such a direction, but at the moment, it's simply a cool-sounding idea with absolutely no experimental evidence at all. Of course, I feel the same way about string theory, though one thing string theory has produced is some pretty useful mathematical tools, so even when a theory is wrong or indemonstrable, it can still be of some use.
The evidence for the multiverse is the math and the theory that implies a multiverse, these theory have been tested and will continued to be tested in ways designed to break them. What's lacking is a way to test the multiverse hypothesis in a way that separates it from other hypothesis. But that's not a fundamental issue with the concept, just the current state of our science.
The evidence for living in a simulation is... the existence of extremely primitive computers. There's no evidence that computers could be powerful enough to simulate reality, nor are there characteristics of the universe that imply a simulation. There's no real reason to think we're living in a simulation other than it's a fun idea.
Worse than that the theory is unfalsifiable, for any test I construct to see if we are in a simulation you can simply reply that the simulation was designed to take that into account. Even if we prove the laws of physics don't allow a computer that could simulate the universe... well you could simply reply that our universe was simulated with different laws of physics.