It could for sure. But that assumes it is set up correctly. And even if it is setup correctly and runs realistic simulations, there are often in real life unforeseen variables like hurricanes, earthquakes, new discoveries, etc.

But how we define what is a fair tax policy is incredibly important. If the goal is not well defined how can an AI reach it?

What about other economic factors. What if a fair tax policy is not complete or possible without some other economic element(s) as a counterpart. What if a fair tax policy could only exist when Universal Basic Income is established.

But let's say that somehow magically all unexpected circumstances are accounted for. And the AI does successfully create a fair tax policy.

There will be those whose stand to loose a large amount under a fair tax policy. They are few, but they are powerful and wield massive influence over government. Would they go along with such a policy? It doesn't seem likely. It seems like even If you get a fair tax policy that is created, there will be people honestly questioning it's validity and people who are against regardless of how valid the result is.

It just seems unlikely to succeed. It may be useful economists might dissect what result it gives hoping to find new insights. But beyond that, I'm not sure how much good would realistically come from it.