The ASAP no expense spared unsustainable way it was done set back space exploration by decades. Geopolitically it may have been correct (SDI certainly was). Apollo is forgivable. Shuttle and SLS are not.
Nixon chose STS over Mars mission because it was presented as the cheaper option afaik. He knew politically NASA had to continue doing something, but he didn't care what they did as long as it wasn't too costly ($ per year, not $ total). After that NASA "lost it's focus" imho.