You can't say something like that without giving good examples.
Despite all the writing problems in the Manga I genuinely think female characters (except Elizabeth) are strong, no princess in distress, unique looking and behaving characters.
Diane even came in last chapter to help her lover and kick some ass despite the difference of power with Maelstarossa. Merlin is fucking strong and autonomous no matter the situation (except with Chandler I guess). Derriere was highlighted from past chapters when male archangels and sins failed, etc...
Diane is strong but her whole character pretty much ended up revolving around her love for King. Her whole “story arc”, if you can even call it that, was loving Meliodas, realizing she loved King, losing her memory, and remembering she loved King again and getting with him. That’s her whole character. He pretty much killed her character to the point where we can say she’s easily the sin people care the least about.
King still has other aspects to his character to the point that once upon a time he was the most popular sin. Diane has always been last among the sins.
Derrerie’s “character” pretty much entirely revolves around her love for Monspiet. I put character in quotes, because I’m not sure if you can even call it that. Wasn’t executed or presented well at all. They had some vague relationship where you could tell they meant a lot to one another. Mel oneshot them. They decided to live together in peace and then turn good randomly. Monspiet died. She died. The end.
Elaine is kinda strong at around 18K, but she isn’t even a character. She solely exists as a reason for Ban to fight, and have internal turmoil about. Literally no other point to her existence.
Elizabeth is just Mel’s girlfriend. Doesn’t even seem to have any importance to her own clan beyond being the SD’s daughter. All she does during confrontations is cry, call for Meliodas, say she’s going to do something but really not do anything, and heal occasionally.
Mera is just generic bad guy number 12, like grayroad.
The only one I think is a well written female is Merlin. She doesn’t cry for others to come rescue her, can handle herself in battle, doesn’t need saving, isn’t obsessed about love, and is mostly confident when confronting most situations.
At least as long as the situation doesn’t involve. Arthur. If it involves Arthur, she melts down. But we don’t know why yet, and the reason clearly isn’t nt because she’s in love with him.