Its also possible she died in transit to the future when the wormhole pulled her back along with the suit (while she was outside of it).
In one of her mission logs, Burnham's mother mentions a 'polyphobic metamaterial' in the suit which protects against radiation, temperature fluctuations, or painful blood clots when she time traveled.
Not only that, but Burnham's mother was pulled back into the future in which Control wiped out all life.
Now, IF Michael's mother survived the trip back through the wormhole AND was somehow protected by the changes in the timeline (chalk it up to travelling so much through the wormhole to try and change history and then ending up travelling through the wormhole outside the suit and being exposed to chronitons which insulated her from changes in the timeline), she would have seen changes happening around her and might have been pulled into the new timeline where life was saved and is living on Terralysium (or is there and dying).
That, or she was stuck in the timeline where Control exists (with the changed timeline now being cutoff from that one so neither future Control nor Burnham's mother could re-enter it), or that timeline was erased (along with Burnham's mother).
There are possibilities here... but not seeing her mother in the promos doesn't say much nor is it an indication of her making an appearance or not.
Its also possible that she will not make an appearance, and may be mentioned in dialogue only.
Burnham's mother will at least be in season 4. The actress playing her wrote on her instagram a few weeks ago she was going in quarantine for Star Trek.