I know that he is pretending to be a girl, but why does he have to look so raggedy. There ...

Damn

tbf the only one they needed to fool is the old grandpa that probably is half blind

I don't even know how he tries to sound like a girl

Lol I've always imagined him with his voice slightly raised(squeaky) but basically the same. Lol. All of his "disguise" seems very bare minimum. Lol

I believe the grandfather knows he's a man, but his grandson loves him. I remember when they said his family has tried many times to fool him before with actual girls. Even in his old age he seems sharp, and just trying to live the reminder of his life in peace, even if he knows it's a lie.

That's really sweet... if he really knows then they're making him dress up as girl for nothing since everyone in that family knows he's a man, poor guy haha I think at the beginning they said someone taught him how to do his makeup too? And they got him only clothes are all flowy to hide his masculine features but knowing that family is filthy rich they really should've given better clothes haha they didn't even try

I truly believe the grandfather's family believes he doesn't know. As much as they love him, he's old, and has been in search of a woman(and her grandchild) that he had some kind of vague connection a long time ago that the family knows died. From their POV, they probably just think he's senile or just a lonely, bored, retired (rich ass fuck) old man holding onto the past for whatever reason.

He knew he was a boy, but he let him stay because he felt petty about him and his insisting on winning him.

Maybe it was just art.. But I believe it was just his hand. Because signs of hard life. Anyway, it was already said that the grandpa has a bad eyesight. And even Yoonsung's father(Myungsoo) noted that his disguise is even better when Hakyung said that Yoonsung might find out sooner of his disguise.
Also Yoonsung's sister-in-law already noted "I thought the whole getup would look comical. But it's not too bad" added to that she even said "Does Yoonsung really not know?". It's because she knows through her husband the whole deal.
I believe at some point Hakyung really does look like feminine in terms of body frame. Also, don't forget that in the current timeline, there are bar customers hitting on him be it a man or woman.
SPOILER AHEAD........................................
................................................................
................................................................
................................................................
................................................................
................................................................
................................................................
................................................................
................................................................
................................................................
................................................................
His grandpa knows about Jiyoung. But he just let the two be together. He knows the goodness in this "Jiyoung" and also it is his revenge to his son Myungsoo. Funny but I really love their grandpa.