If a production exhibits diminishing marginal product, the slope of the corresponding total – cost curve will become steeper as the number of output increases.
The law of diminishing marginal product shows an increase in one production variable while other production variable remain constant will initially increase overall output but will generate less output as more of that variable is added.
This simply means, increasing one variable will only increase output to a certain point.
As quantity of output increases, the curve becomes steeper and not flatter or linear.