I'm assuming the signal you want to slow is an analog voltage representing the throttle position. A center tapped potentiometer where the center tap is the output and the other ends are connected to two different voltages, for example the skate board battery + and - terminals, would be an example. That's what I have in mind when I'm writing this:
I credit Wikipedia for the schematic symbols that I copy and pasted to make this.
I'm guessing you want a smooth response to the throttle so the skateboard won't jerk when you change the throttle fast and make you lose your balance. A simple way to do this is a RC circuit.
An RC circuit has a time constant which is traditionally given the greek letter variable tau. That's the horizontal axis in the graph on the schematic.
tau(seconds) = resistance(ohms) * capacitance(farads)
The rate at which the output voltage changes will be proportional to the difference between the input voltage and the output voltage. This means if you jump from 0% throttle to 50% the voltage will increase half as fast as if you jumped to 100%. If you jump to 50% and your tau is 1/4 second, then after 1/4 second your pwm duty cycle will be 63.2%*50% = 31.60%. This is assuming the rest of your circuit is designed so that 100% throttle results in 100% duty cycle. You can read the graph to see what it will be at each multiple of tau.
Things get more complicated mathematically with more than an instantaneous change from one voltage to another. The differential equation is in the wiki article if you want it. Just know that the higher tau the slower the response. Too much and you'll feel like your board is sluggish and unresponsive. Too little, which is what I think you're trying to fix, and you feel like the controls are too jerky.
Some things to keep in mind when picking your R and C. There are infinite combinations of R and C that will achieve the time constant you want. However too low values of C and too high values of R will lead to the unideal behavior of the rest of the circuit having too much say in your circuit's behavior. The opamps I put in there are unity gain buffers and help isolate the RC circuit. Op amps have low output impedance and high input impedance. I think the most important factor will be the input impedance which should be > 1M Ohm. You can get away with a cheap op amp here because the demands aren't high.
I suggest starting off with a 47k resistor and a 4.7uF capacitor. This will give you a tau of about .22 seconds.
47 × 103 Ω × 4.7 × 10-6 F = 0.2209 s
You can play around and decide what you like. Once you get a good default value you can replace the resistor with a variable resistor and have a knob that controls the responsiveness on the fly. Think of one end as comfortable and the other as performance. You can put a constant resistor in series with the variable one to set a minimum smoothing if you want to keep it from throwing you off if you turn the knob all the way down.
A note about the op amps. You want the positive voltage supply to be greater than your highest input voltage and your negative voltage supply to be lower than your lowest voltage. If you pick a rail to rail capable op amp then you can have them equal.
I suggest the ground on that capacitor be the lowest voltage available. This will allow an electrolytic to be used if the negative terminal is connected to ground.