Using SMD capacitor as a decoupling capacitor
I think you have a misunderstanding about capacitors. The physical shape does not change the properties if the ratings are the same. So a 10nf 30V SMD cap is the same as a 10nf 30V through hole cap.

A decoupling cap in power circuits is used to short high frequency noise to ground. You sometimes have a few of them on your board near IC's. This prevents unwanted spurious signals from messing up your digital or analog signals.

A decoupling cap might also be used to filter out the DC component of a signal and only allowing the AC portion through. This is commonly used for analog inputs to amplifiers and RF circuits.