Mass requires reaction (generally: fuel) both to accelerate and decellerate. Absent alternative delta-V mechanisms (usually: aeroraking, as lithobraking is perceived as generally too extreme), this rapidly runs into the tyranny of the rocket equation.

<https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation>

Even without earth-to-orbit costs, that mass has real costs, and reduces available payload.

If that shielding mass can be dual-use (e.g., water), reactive (e.g., electromagnets), or reduced to a very small amount (emergency shelters, wearable garments), it becomes more practicable. That still doesn't make it easy.

There's also a discontinuity in radiation exposure. Passing through radiation belts (e.g., Earth's van Allen belts, or those around Jupiter), and solar storms, are both predictable and special precautions can be taken. Cosmic ray radiation is unpredictable, high-energy, and is far harder to guard against. Some risk is inevitable.