He is just a nice efficient threat and generally you don't want those making your life difficult
The biggest part of sideboarding I want to stress is what to take out and when
Universal Package (23 cards, roughly $12) Mana-fixing artifacts: Armillary Sphere, Darksteel Ingot, Fellwar Stone, Spectral Searchlight, Manalith, Mycosynth Wellspring Utility artifacts: Sun Droplet, Illuminated Folio, Urzas Blueprints, Crystal Ball, Temple Bell, Vensers Journal, Steel Hellkite, Culling Dais, Spine of Ish Sah Equipment: Bonehoard, Nim Deathmantle, Mask of Avacyn Lands: Evolving Wilds, Rupture Spire, Terramorphic Expanse, Shimmering Grotto, Exotic Orchard Commander is all about having fun, and you cant have fun if you cant play your cards, so you need mana fixing, mana acceleration, and card drawing to keep the gas coming
Finally, a 6/6 with flying and trample is nothing to sneeze at, and it can provide card advantage over time as it recurs cards and grows even bigger with its triggered ability
Of all commanders discussed, The Mimeoplasm is likely the most powerful
It has consistently been the highest selling Pokemon card and has received the most fan and media attention