The short answer is: it can, and often will, if the planter stays in one spot long enough. An urban planter box set directly on a lawn does not usually kill the entire lawn, but it can definitely kill the grass directly underneath it. Grass needs sunlight, airflow, and a healthy balance of moisture and drainage. A planter box blocks all three to some degree.
The most common problem is smothering. When a planter sits on grass for days or weeks, the grass underneath is shaded out and gradually turns yellow, thins, and dies. The effect is worse with larger, heavier, or solid-bottom planter boxes because they cover more grass and trap more moisture. Dark-colored planters can also heat up in the sun, creating a hot, humid pocket beneath the box that stresses the grass and may encourage fungal problems.
Moisture is another issue. Water draining from the planter may keep the lawn underneath constantly wet, while the box itself prevents air from reaching the soil. In other cases, the planter can create a dry patch if it blocks rainfall and you water only the planter, not the grass around it. Heavy planter boxes can also compact the soil, making it harder for grass roots to recover after the box is moved.
So, will it kill your grass? If you place an urban planter box directly on the lawn temporarily and move it every few days, damage is likely to be minor and the grass may recover quickly. If you leave it in the same spot for a season or longer, the grass underneath will probably die, and you may be left with a bare, compacted patch. The lawn around the planter usually stays alive, but the area directly beneath it is at risk.
To protect your lawn, avoid setting the planter directly on grass for long periods. Use a base with feet or casters to allow airflow, place it on a hard surface such as pavers or a gravel pad, or move the planter periodically so one patch of grass is not smothered. If you must keep it on the lawn, choose a well-draining planter, manage runoff, and check underneath every week or two. If the grass starts to yellow, move the planter before the damage becomes permanent. With a little planning, an urban planter box can still look great without sacrificing your lawn.