Is Tatsumaki as strong as Saitama?
It has been shown that Tatsumaki's powers destabilize, and to an extent become unusable, when she suffers a concussion after Psykos ambushes her from behind.
She was also strong enough to lift Saitama in the air while in a weakened state, although she was only able to lift him a few feet in the air.
Is Tatsumaki a bad guy?
Well, long story short, people in the One Punch Man should be VERY happy
Tatsumaki is not a villain. She possesses incredible psychic/telikinatic powers, and over the course of series was shown manipulating and levitating objects in very large area around her - seemingly effortlessly.