According to the Hero Association ratings, several characters have more power than Tatsumaki. Bang, Atomi Samurai, Flashy Flash and amazingly King are rated to have a power of 10 while Tatsumaki only has 9. But, Tatsumaki was capable of taking a monster who was probably a demon level in one blow. Why does the Hero Association ratings have several S Class Heroes to be more powerful than Tatsumaki?
It's possible that the translation of that stat as "Power" is a (slight) mistranslation. I have seen it alternatively translated as "Instantaneous Power/Speed". If we assume that is a more accurate translation, then it means it's a measure of how quickly they can utilize their power/speed. As such Tatsumaki's rating may be lower than max if it takes her a little bit more time to unleash her strength; at least as far as the association thinks/knows anyway. Martial combatants like Atomic Samurai can use highly trained reflexes to react powerfully with little to no thought, while Tatsumaki, presumably, needs to actively think/concentrate for most situations. This seems supported by an important plot point later:
Espers must choose between offense and defense, as they can only use one power at a time. This creates an exploitable gap where you can land an attack while the Esper is trying to attack. This pops up near the end of the Garou/Monster Association arc. We are also told during that sequence that her barrier remains up and effective even when she's unconscious (and apparently concussed). As such it seems to be Tatsumaki's default state to be in defensive mode, and she must consciously and actively switch to an offensive attack.