That's Finnish. This is something a teenager would grumble to any comment or question made to them.
I think it derives from an old stock reply, I recall my generation using, when we were kids.
Someone said something to you like: "You're stupid." -
Olet tyhmä.
The other one replied: "Your mum was, when she was having (making to be exact) you." -
Äitis (mutsis) oli kun sua teki.
So this has later apparently shortened to "Your mum was." -
Mutsis oli.
If you want to be polite though and use proper grammar, "your mum was good" would be
äitisi oli hyvä.
