This word does not have a negative connotation. The usage of some of its meanings is less popular nowadays (like the one in your example), while the most popular meaning (something that does not make sense) is also the most recent one (hence a wrong assumption that this word relates to a teenage slang). But nonetheless you can certainly utter this word in a polite company.
"Муть" has several meanings: (1) the most direct one would be describing an unclear precipitation in water, e.g. "какая-то муть в воде плавает"; (2) other meaning could be for something in a foggy state, e.g. in nature, more commonly used in a form of a stemmed adjective "мутный", e.g. "мутная река"; (3) you can also describe a lack of clarity, e.g. "мутная голова", (4) or sadness in your state of mind - this meaning would be the most applicable to your example "Без тебя такая муть", and finally, (5) the most colloquial usage to describe something confusing, which does not make any sense, e.g. "Эта задача такая муть". But even in the last meaning the word does not express scorn or disrespect, it is not rude by itself, indicates mostly confusion and lack of understanding. Rudeness can come from the way a person reacts to any such situation, within some context (to what most people refer here), but it doesn't have to do with the language itself and more with used facial expressions/gestures/whatever else is said.
Here is the first googled example in the literature's usage, indicating that this word is not only colloquial and surely not limited to slang, a snippet from Marina Tsvetaeva's poem:
Хочу у зеркала, где муть
И сон туманящий,
Я выпытать -- куда вам путь
И где пристанище.