I respectfully disagree. To say that God does not care about any of our actions is an insult against his omnipotence. God cares about every little detail in our lives.
On a more specific level, as for kneeling, standing, sitting, etc during certain prayers, Jesis said we must make ourselves as children, ie we must be humble. Our Western cultural traditions, gong back many centuries gives us kneeling as a outward display of humility. Not just our thoughts, but actions are important to God during prayer, our body is part of our humanity, so it would be wrong to say that an outward display of humility is not important.
Also, God cares about obedience, and here in the US, the Bishops give us explicit instructions on when we should kneeling at mass.