Is it ok to put decoupling cap over analog input of amplifier?
Probably it will work. All effects are very small there. 

Still, from best practices point of view, this not a good idea. The decoupling capacitor provides energy for current peaks, which for class D amplifier are pretty high. 

Edit: yours is class B, so frequencies are low and hence effect are even lower. 

So you will have a trace with varying voltage for capacitive coupling with the signal and pulsed currentl for inductive coupling. Either way the switching (edit: for class B audio)  frequency will get to your input. 

Why don't you use bottom side of the board for GND?