Do I need to add smoothing capacitor for this setup
Never underestimate the benefits of capacitors. If the lighting was constant intensity any capacitors would be redundant, however if your strobing the LED's or sequencing them, small brief voltage drops would occur in the power feeds. Even if the WS2812B drives the LED's with constant current 100uF 16vdc electrolytics will remove glitches from the power feeds, especially at the LED's furthest from the power source. If the flashing of the LED's seems to affect other LED's you can use 1,000uF 16vdc capacitors at each LED driver. Capacitors are cheap for the benefits they offer. Note that the 100nF value you chose are still useful as decoupling for high frequency noise, best if placed at the power pins of the IC.