Please don't put a tree that has been in shade straight out into full sun. Leaves are like our skin and can change and adapt to different amounts of sun.
Putting either you or a tree from shade into strong sun all day will result in sunburn.
Move a tree from shade to sun gradually increasing exposure over several weeks if possible to reduce sunburn.
Sunburnt leaves is not fatal. New leaves will grow that will be adapted to the new regime so if you have made a sudden shift or have no other option the tree will look sad for a few weeks but will usually survive.
I agree that this one looks like not enough sun and that sun is better for Chinese elm but there is one variety - Catlin elm - that naturally has long, thin shoots as shown and no amount of sun will change that natural growth habit. Catlin is much slower to thicken and ramify but regular trimming helps.
It is difficult to tell the difference between over water, under water and sunburn because all 3 reflect dehydration of leaves. Under water is self explanatory. Over water causes roots to rot and then there are not enough roots to transport the water to the leaves so the leaves dehydrate despite plenty of water in the soil. Sunburn may partly be not enough water for the leaves to stay cool because the heat evaporates water faster than the tree can supply it. Wind can also cause similar dehydration of leaves.
Only by knowing previous history or checking the roots can you tell on from the other.
If you've kept the soil constantly wet for more than a few weeks suspect over watering.
If you've kept the soil relatively dry because you are terrified of over watering suspect under watering ,especially if it shows up in spring or summer as days get hotter and drier.
If you've moved to a sunnier location suspect sunburn.
If you've had hot, dry winds recently suspect dehydration from wind burn.
Another factor for Chinese elm has been raised in several similar threads. Chinese elm don't always drop leaves in winter. Catlin elm is more evergreen than other varieties but leaves still only live for a single year. If they did not drop in winter they are still at end of life so will turn brown and die of old age in spring. This only affects lowest, older leaves and new leaves look healthy.
Curling leaves can be from insects or dehydration. I cannot see anything to worry about in the bits of photos you've posted. Unless it gets worse not enough to panic.