Technically, social mobility is just relative income position changes over time, which applies to zero sum games. It does not imply that you can significantly increase your income. Social mobility is also an indirect measure of income distribution, and by proxy geographic compactness -- you can double your income and still have low mobility if the distribution is wide.
A country can be "highly mobile" with flat GDP and limited income growth opportunity for the individual. That is just how the math works out. Social mobility is a term-of-art that does not mean what most people intuit it to mean.
I think it's more apt to consider mobility in absolute terms in this context as opposed to relative. Having a country with super high mobility but extremely low GDP is obviously not favorable.
A country can be "highly mobile" with flat GDP and limited income growth opportunity for the individual. That is just how the math works out. Social mobility is a term-of-art that does not mean what most people intuit it to mean.