PWM Dimming results in input ripple due to excessive inrush current
Try adding another inductor identical to L1 on the input, and use as much capacitance as you can fit in between the two inductors.

You could also reduce the output capacitance C12 to reduce the amount of energy that needs to be supplied every new PWM cycle. This will increase your output ripple, and eventually become unstable, so take care there.