Yes, mulching the bed should improve results. What general type of plants are you planting? If these are flowering annuals you should dig in organic matter throughout the bed before planting. And give them some mulching. If these are instead flowering perennials it would be better to plant in existing, loosened soil only and then mulch well afterward, at least once inch deep.

In neither case is replacing the existing soil with different soil in individual planting holes a good idea (this procedure often has adverse effects on how water enters and exits the planting hole). And in both cases mulch should be kept well away from the stems of the plants.

If existing soil seems completely unsuitable for kinds of plants being attempted then it may have to be excavated and replacing entirely over the whole bed-sized area with a different soil. Or a different soil can be placed on top of the existing soil, as a berm, mound or raised bed, and planted in. Without blending the two soil layers together (which would defeat the purpose of the new soil layer).