Is there an idiom to describe someone who is really skinny and whose clothes don't look good on them?

It is also common to say that someone looks like they are "swimming in" their clothes, if they are way too large for him or her.

At least in the US, to "look like a scarecrow" would be the most common idiom that is widely understood. Some examples going back to at least 1905:

I'm really sick of all these actresses looking like birds... I'd rather look a little chubby on camera and look like a person in real life, than look great onscreen and look like a scarecrow in real life. - Actress Jennifer Lawrence, 2011

Like all the people in the loft she dressed like a hippie, and being so skinny her loose-hanging clothes made her look like a scarecrow swaying in the wind. - Kokis & Wilson, The Art of Deception: A Novel, 2002

The lad grinned sheepishly, and began to hustle into his garments. They were a world too large for him, and hung upon his shrunken limbs in a baggy and outlandish fashion. His shoes were ten sizes too big; his cap rested upon his shoulders. "Huh!" he muttered in disgust. "I look like a scarecrow." - James Ball Naylor, "The Little Green Goblin of Goblinville, 1905.

So the idiom goes back a while but is still in use in popular culture today.

My rather colorful aunt has a few sayings for this:

• “Like two towels on a toast rack”

• “Like a scarecrow”

• “5 pounds of potatoes in a 10-pound sack” (and the inverse for fat people in small clothes, “10 pounds of potatoes in a 5-pound sack”)

But I don’t know that these are necessarily well-known idioms.