"How is mom?"
It is correct. You would only use "Habari ya mama." when speaking about her, not TO her. (That would be something like "Habari yako?" or "Hujambo?") "Habari ya..." usually is used to ask the person you speak to about various events/places/people - roughly such as "How is... (it going with)?" - it is to get news about something specific/or see below: to be polite.
ya is singular and za plural So for example ndizi ya mama means the banana of mum and ndizi za mama means the bananas of mum. It refers to different classes in Kiswahili, for the case of habari and ndizi which are in the same class, the plural looks the same as the singular, the only differentiation you can have is through stuff like ya and za (sorry I am not a grammar expert and I don't know the exact words for it).
I hope I could help :)
Thank you for your answer. :) I have googled the ya/za and I have found this: "There are two possibilities : either you are expecting a precise answer : in this case you use Habari ya ... ? (= What's the news of ... ? ), or you are simply asking out of mere politeness, without any precise intention, in that case you use Habari za ... ? (= What are the news of ... ? ). " What do you think? :)