Are mashup copyrighted content

I see many channels with mashup music, most of them monetized. I wanna know if their adsense account wont get banned?

are you sure it's monetized? it may just be the ads that are automatically added from the content ID and which go toward the copyright owner.
most music isn't able to be monetized without altering its properties.
i'd say they'd most likely get away with it though if it's not obvious enough for copyright holders to find, but other than that, mashups of clips or pics or gifs etc. are usually fine to monetize unless they're of movies or tv shows or something.

They might be got prier permission from the content owners or profit sharing, mashap properties are copyrighted, it is risk to use.

it will get banned from legit people who surf for videos like yours and flagged it.I heard theres a group on reddit that does that.They are so badass