Flag Smashers were fine. This show wasn't focusing on the villains, it was focusing on Sam, Bucky, Walker and to a lesser extent even Zemo.
Flag Smashers were mooks to punch with a noble enough goal that you could see both sides of it so Sam could do the Captain America thing, Walker could get the serum, Bucky had people to fight, and Zemo had people to murder lol.
They could have been a bit better if Karli actress had more screen presence. When you get small amounts of screen time, you got to smash it and she simply didn't. They wanted to blurr the line between "psychopath terrorist" and "young teenage idealistic girl" and what they got was something bland in between. She never felt menancing when she needed too and I never really felt terribly sorry for her during the "sad" stuff.
If you throw out the Sharon plotline (just keep the Power Broker in the background) and that time and use that ot show us what things were like during the decimation and then blip... it would be easier to feel sorry for Karli and whoever she was fighting for.