Tables on Mobi are quirky to say the least. I've found from my own books that, for example, explicitly specifying cell widths will sometimes work and sometimes not, seemingly completely at random. Centring - both of the table as a whole and of cell contents - should certainly work OK.
Best not to use CSS, because Mobi doesn't support it - all that's happening with CSS is that the Mobi creation tool is trying (more or less successfully) to convert it to HTML tag attributes, so it's better to just code the attributes directly.
|