The six knights of black were nowhere near the power of the T.C, in fact the weakest T.C is probably more powerful than all of them combined. It was said most likely to keep the viewers interested in the movie. You would easily lose interest if you knew that they were a weak bunch that could be handled in less than 10 seconds. This is also the reason why they had to pull crap like Meliodas is out of commission due to the sword, etc. I'm not gonna give too much of the movie away here for those of you interested in watching the movie.
As to how strong Mama Hawk actually is,
This is not a huge spoiler but nonetheless it's still a mild spoiler.
Mama hawk is actually a creature from the purgatory just like hawk, she's pretty damn strong. I think she's stronger than most of the commandments in terms of raw strength, she can't be actually compared to them because the commandments also have the power of the curses bestowed by the demon king. I say this because she's able to withstand an attack from Derieri and she's able to eat Monspeet's hellfire bird like it was nothing. As it's reveled later on in the series, all purgatory creatures are pretty damn strong because it's a kill or be killed situation in there, this is also the reason why hawk is alive despite being killed by black snow in season 1.
The next part is a really big spoiler, read at your own risk.
Later on in the series it's revealed that hawk has a brother who's also from purgatory and he's a bad ass who's actually pretty damn strong. I'm not gonna give away too much, but based on that mama hawk is a very strong creature. But the actual power levels aren't compared anywhere.
If the answer still hasn't satisfied you, continue to read. But this could spoil a pretty big surprise in the series for you, I only try to justify why I think mama hawk's a powerful creature in the next part, you may skip it if you wish to enjoy the series.
Later on we find that the demon king is the one who sent hawk, his's brother and mama hawk to spy on Meliodas. So I'm guessing he chose the elite of the creatures to do so and gave them some kinda power enhancement so they could fulfill their job and not die like Wandel(the parrot sent to spy on Meliodas by the demon king), which makes them the strongest of all the creatures of purgatory.