As the other members have said, there is no political connotation to the concept of 'mainland China'.
The description is a geophysical one - it refers the regions of China which are physically part of the continent.
We use the same term for other parts of the world with no political overtones, either. For example, we say 'mainland Europe' to refer to the main land mass of the continent. This excludes, for example, Corsica (which is part of France), Sardinia and Sicily and other small islands (which are part of Italy) and the Balearic Islands (which are part of Spain). Sovereignty doesn't come in to it - these places aren't mainland Europe due to the simple fact that they are separated by water.