Repeated small discharges and charges in themselves will not damage the battery - contrary to popular belief it isn't necessary to fully drain the battery at all, and in fact it is better to keep the battery at least 40% where possible.
Wireless charging produces more heat than wired charging, however - and heat is damaging to Li-Ion batteries. I've not actually used a wireless charger so I don't know exactly how hot it gets - but I know it is considered one of the main drawbacks to the whole concept of wireless charging. It's ok to leave it on there, especially as the mat should cool down once the phone is fully charged, but you might shave a little off your battery's lifetime. Which is to say, your daily capacity shouldn't be affected too much, but a few years down the line the battery may die a bit quicker.