I don't see why manual trim is needed in the sim? Its in trim when you calibrate at the start and there is no control loading for you to trim out.
The flight physics are already phenomenal for a mobile sim and by adding manual trim you will just fight against that somewhat.
The trim adjustments you see Boeing pilots make on approach are so small that a slider UI would probably not do it justice anyway.
I do agree some aircraft start out of trim when you spawn on the approach, I think the 737 is sometimes bad for this, maybe that could be rectified in a future update.