Ying is a tricky champion but you can get a huge amount of heal with her if you play well.
Try to place your "Illusions" in the way that enemies can't hit them and that your allies can get healed by them. Be careful to sometimes place them in places that are safe too if you need to use "Dimensional Link".
A great placement is : one "Illusion" on the point, one "Illusion" on a flank route that my flank is using so it can be healed by the "Illusion" while you're healing the team!
Do not stay static, try to move a lot as you're an easy target for flanks or even damages. Try to stay close to your frontline. If they die, try to play safer and place your clones in a way that they can heal you too and that you can teleport to them if you really need. Don't be afraid to use your ult to teleport to your allies if they need your help and that you're way to far away or if you need to escape super quickly and you can't die because your team absolutely needs you in that situation. If you rush "Morale Boost", that ain't a problem.
Main build with Ying
Shatter instantly Heals your target for 800, but its Cooldown is increased by 1s and it no longer explodes your Illusions. Reduce the Cooldown of Shatter by 2s if you miss.
Reduce the Cooldown of Shatter by 2.5s.
Increase the duration of your Illusions by 3s.
Reduce the Cooldown of Illusion by 1.5s when an Illusion dies to an enemy.
Increase the Health of each of your Illusions by 200.
Reduce your active Cooldowns by 2s after getting an Elimination.
"Squadron" can be swapped with "Fracture".
This deck provides you great source of CDR. "Encouragement" is great to have CDR when you get assists.
Increase your Ultimate charge rate by 30%.
Reduce the Cooldown of all your abilities by 30%.
Kill to Heal
Getting a kill or elimination heals you for 600 Health, unaffected by Anti-Healing.
Reduce the duration and effectiveness of Crowd Control and Slows by 75%.
Reduce the Damage you take from Direct and Area of Effect attacks by 18%.
Her ultimate is super strong so you have to rush "Morale Boost" in order to get it super fast. As your "Illusions" heal your team passively, you'll get a lot of assists so "Kill to Heal" is a great healing option to maintain health while your team is doing kills. "Chronos" is always a good choice on healers but Ying doesn't need a Chronos rush as her cooldowns are pretty low and she needs her Ultimate more often.
"Haven" is always the go-to choice in Defensive items. "Resilience" is an excellent choice against hard crowd control spells / champions.
Ying benifits from champions that can get out of the way to avoid the anti-healing or champions that are mostly static and stay in your "Illusion" healing range.
Champions that can easily remove the anti-healing is a great thing as Ying suffers a lot from this. "Rejuvenate" is always a great pick for Tanks, Damages or even Flanks if they need.
"Bad" synergies for Ying are champions that tend to fly or move a lot, they'll only benefit from your RMB "Shatter" heal and not much from your "Illusions". This is not a reason to not play Ying but be careful while playing with champions that fit this "flying and moving a lot" gameplay as it'll be kinda hard to heal them sometimes.
As all the healers, the counters for Ying are Flanks, and especially flanks that can easily follow her even if she uses "Dimensional Link". Try to play around your team and your Tank, make call-outs if you're playing as premade.