I'd never heard of Sinistar before, but this is hilarious.
You're playing this relatively normal-looking game with a little Asteroids-style ship and all of a sudden, a giant frikkin' head-thing shouts "I LIVE" and starts chasing you while shouting creepy phrases and roaring? Hilarious, and actually a bit scary.
I think my favorite part of the design is the eyebrows. Somebody posted a video with silly audio overlays, and I kept cracking up when Sinistar waggled his eyebrows in time with some of the music.
Also, I am oddly reminded of Tornedron from Transformers.