Yeah, I agree; the large spots suggest sunscorch. Are they getting afternoon sun?
Hydrangeas, in their native environments, are understory shrubs. If their leaves in direct contact with the sun start displaying these large swatches of color change but the leaves not in contact with the sun remain dark green, please research just how much sun (and when) the leaves are getting. Transplant if needed. Or if you prefer to transplant when dormant in Fall/Winter then cover them with a chair or umbrella until the Fall and then move it.
In the northern states, where the sun is weak, mophead hydrangeas can sometimes be placed in almost full sun conditions but... the further one goes south, the less sun they like to have. In general terms, they need (a) dappled sun, (b) morning sun, afternoon & evening shade or (c) full or bright shade. Down here near Dallas/Fort Worth, shady space is in short supply but I give hydrangeas morning sun only... and shade starting at 11am. Or full but bright shade.
Some insect pests or the cat may have nibbled on the leaves. I would try to make sure that the leaves do not touch the soil if you can. But the damage looks minor and aesthetic only so I would not worry unless you see signs that there is new and more damage. The pest may have nibbled and flown away by now, so to speak. But if you see new damage, investigate what is causing it by visiting several times during the day and evening hours, when most culprits show up. Look under the mulch and rocks.
The spots on the stems are normal; you can ignore them.
Water the shrub -say around 1 gallon per watering- by watering only the soil near where the crown is and moving outwards. That is where the rootball is and where most of the roots are located right now. Water the soil early in the morning, around 6am. Do not water the leaves to prevent fungal issues. Water when the soil feels dry to a depth of 4"; try to get the soil moist down to a depth of 8" after a watering. If your soil is sandy, add 50% more water.
The mulched area looks fine and was shady when the pics were taken. Keep the soil as evenly moist as possible.
Summer is encroaching; I am already in the 90s. Hydrangeas react to high temps by wilting. Usually happens when temps get to or exceed 85F. It can also happen if it is very windy. The plant looses moisture thru those big ole leaves faster than the roots can absorb the moisture thru the roots. The blooms can also be wilted. But eventually, the plant's roots catch up when moisture demand tapers off at night and the plant will look fine in the morning. The catch is this requires that the soil be maintained as evenly moist as you can. Not wet but moist. No periods of dry then wet then dry then wet. Wilting is a problem in the first or first few summers. Once the plant becomes established and develops a larger root system, these wilting episodes diminish but may not go away completely, especially in really hot parts of the summer. So, I like to keep an eye and visit my hydrangeas in the mornings. For example, this morning I noticed one unhappy camper on the way to get the newspaper so, I watered it (all the others looked fine).
The finger method can help you keep under and over watering in check. Use the finger method in the mornings, at about the same time, daily for 2-3 weeks. When the soil feels dry or almost dry, give the plant water (1 gallon). After each watering, make a note on a wall calendar. After 2-3 weeks, review the watering info and average out how often you had to water. Say, every 3 days? 4 days? etc. Then set the drip irrigation or sprinkler to water the shrub 1 gallon of water on that same frequency (every 3 days or days, etc). If your temps then go up/down 10-15 degrees and stay there, use the finger method to see if you need to tweak things. As temps regularly warm up, you may need to water more often or try watering more (go from 1g to 1.5 gallons). Then as temps moderate in the Fall, revert back down. In the winter, you can reduce waterings further once the shrub has gone dormant. By then water once every week or every two weeks if dry. Stop watering if your soil freezes but resume when it thaws in the Spring.
I hope all that helps?