Hi, No, "seorang perempuan" means "a woman (or more precisely a female)"..... I guess that in a more correct way of talking, it would be better to say "ibu saya adalah seorang perempuan". But here (in Indonesia) we tend to shortcut everything we are saying..... So that's what it is..... (I'm not a native Indonesian but I never heard someone saying "adalah", so I'm not able to provide you further explanation! Good luck! )
"seorang'' is an optional, just to make sure that there's one person. "se'' here can be translated as ''a''. And ''orang'' means ''person''. Another example of ''se-'' is ''sebuah ( means a fruit / a piece)''
And you're right! "adalah'' sounds formal and we rarely use it in daily conversation. It means ''is'' or ''are''