I've got to disagree there. While a lot of the game focuses on random elements you can use the environment to a large advantage.
Since you can outmaneuver all other ships by turning on a dime, you can use the planetoids around your ship to 'check' your opponents. So the skill element becomes how fast you can break out of position and find cover. This becomes a vital skill by the Warrior Zone.
You can also shoot down enemy shots. While it can be maddeningly hard (maybe even impossible) to do it reliably, it still adds a strong element of skill in being able to make that 180 degree turn and take down incoming fire.
A game like Defender may give you more "wiggle-room" on skilled play, but Sinistar certainly isn't devoid of it, it just is obfuscated behind a wall that looks a lot like luck. Robotron is a closer match due to the "trance" or "zen" style play that both games exhibit.
That said, if you can get to the Void Zone, all bets are off since you're pretty much in open space.