Bond Angles and intermolecular interactions
When people talk about bond angles in molecules, they are usually talking about the equilibrium bond angle in isolation. This can be derived from the energy minimized structure of the molecule in isolation.

As you point out, the angles in a real, non-isolated molecule will fluctuate due to interactions with neighboring molecules. However, the actual angle formed by three successive atoms in a molecule will naturally fluctuate even if the molecule is isolated and has some kinetic energy. Everything's always jiggling around.