When referring to a general mother, I'd always just use lowercase.

> 
> Our mums went shopping together. Your mum bought a new suitcase.
> 
> 
> 

However, when addressing my specific mum, I couldn't decide whether or not to capitalize the M.

> 
> You'll have to adopt that dog **M**am; we want to see it.  
> 
>  When are you coming to England mam?
> 
> 
>