Is it normal to have this long fall time with a MOSFET and a LED strip at 100Hz?
As I would have guessed, you're measuring across the LED/resistor. Be aware that the LED (when it goes out, as the voltage falls below its conduction voltage) becomes a very high resistance. This, is conjunction with the FET's drain capacitance, forms an R-C circuit with a rather long time constant. 

Which is exactly what you're seeing. 

Indeed you can pretty much identify the voltage at which the LED is fully off from the point where the negative edge ceases to be vertical.

So, for the circuit as revised, yes this is normal.

Put something like 10 kilohms across the LED, and you'll see a much cleaner waveform, as an experiment. (Leave it there if you like, it won't waste much power, but that long fall time isn't doing any harm)