Is reading the Qur'an, performing dhikr, or sending blessings upon the Prophet, while in an imbalanced sitting posture (such as crossing one leg over the other, putting one's hand in a pocket, or sitting as if half-asleep), considered a form of mockery or disrespect?

We advise you to turn away from whisperings (waswas) and not pay attention to them, for indulging in them leads to undesirable consequences. There is no disrespect in remembering Allah and reciting the Quran in all circumstances. The Messenger of Allah, peace and blessings be upon him, used to remember Allah at all times. Allah praised those of understanding, saying they remember Him "standing, sitting, and on their sides."