Frankly, any monuments built after the Civil War don't need to be in a museum. What historical value do they really have that photographs wouldn't be a sufficient alternative for? These were statues built by "lost causer" white supremacists long after the war to perpetrate false myths about why the so-called confederacy lost. Since they weren't built during the war, they're not historical artifacts of the war. They're just relatively recent symbols of racism and hate.
Monuments and statues built during the war? Well, I haven't been able to find any info on whether any such monuments even exist, but if there are any, then yeah, those have historical value, because they were built by the people who lived through the war, perpetrated it, and participated in it themselves. Of course, if there are any such monements today, they shouldn't be standing outside of a museum, but they do have enough historical value as direct artifacts of that war to be placed inside a solemn museum of reflection on a sinister time in United States history, I think.