I'm on the Decepticon team and my teams have been crushing Autobots like tin cans for the most part. Maybe you're just going up against the really good Autobots? Do your teammates keep disconnecting on you (that's happened to me a few times, not an unwinnable state but definitely a major disadvantage)?
Or maybe you just haven't found the right weapon for you yet. That happened to me the last couple of Splatfests, so just switch up your weapon & gear loadouts until you find one that works for you, then run with it.
As for Splatfest itself, it's probably not rigged. Just don't worry about team-wide win/loss ratios, you'll do better that way.