on passing, 저질 can be also used as an insult to people, generally meaning "low-life, vulgar"

"ㄹ" is just one of Korean alphabet characters, so it itself doesn't have to do with good or bad pronunciation. I've heard Korean "ㄹ" is pretty much close to the dark "L" sound in American English, like in "milk", but the exact sound would be different. You can not but repeat native Koreans to master its pronunciation.

저질발음 = poor pronunciation

ㄹ is kind of a combination of the letter l (el) and r. Sometimes it sounds more like an el, sometimes it sounds more like an r. It's a difficult letter for English speakers to pronounce. I looked up 저질발음 in the dictionary and 저질 means "low quality" and 발음 means "pronunciation" so all together it means "low quality pronunciation", I guess. Which is perfect, since we English speakers have trouble with 'ㄹ' as you said. Haha