Saving is always a good thing. Why is it bad?
1. Traditional conservatism dictates that it's our money, not the government's. Instead of a sovereign wealth fund, the proceeds should either be applied against the debt (not the deficit) or returned to the taxpayers. Private investment is more efficient than public investment.
2. The sheer size of a US sovereign wealth fund - if, for example, it included the SS Trust Fund - would compromise overall market efficiency, making such markets more volatile.
3. Sovereign wealth funds are notoriously inefficient and corrupt. They are incompatible with democracies, if we still have one.