Applying mulch around new plantings (as well as in established beds) is ALWAYS a good idea. Mulching is an excellent way of keeping weed growth down as well as slowing evaporation of soil moisture. Just make sure the mulch is not piled up at the based of the plant.

Mulching is a good idea. But 'meant to be' makes it sound as if there are rules about it. Your Hydrangeas won't die without mulch. To avoid grass growing around them is just a question of doing some proactive gardening. The ground should have been made weed and grass free before the shrubs were planted. Adding mulch means you'll have to do a bit less weeding, but it won't eliminate the need all together. And edge the beds sharply to avoid grass growing into them.

And, remember, 'mulch' can be from a variety of materials. If you have leaves, you can put them undert the hydrangeas as a mulch. You could put cardboard and cover with something pretty. Compost makes a lovely mulch.