I am new to TPLink, just bought some Deco X60 and registered a hostname with the ddns service.

TPLinkdns.com has 2 nameservers:

;; ANSWER SECTION:
tplinkdns.com.          900     IN      NS      ns2.tplinkdns.com.
tplinkdns.com.          900     IN      NS      ns1.tplinkdns.com.

;; ADDITIONAL SECTION:
ns1.tplinkdns.com.      86400   IN      A       52.204.177.89
ns2.tplinkdns.com.      86400   IN      A       54.87.217.253

But only one of the servers resolve? Is this normal or just outage today? Can tplinkdns.com be trusted at all?

Hostnames has been shielded to protect the guilty ;)

Yes:

$ host XXXX.tplinkdns.com 52.204.177.89
Using domain server:
Name: 52.204.177.89
Address: 52.204.177.89#53
Aliases:

XXXX.tplinkdns.com has address 31.xx.xx.xx
Host XXXX.tplinkdns.com not found: 3(NXDOMAIN)
Host XXXX.tplinkdns.com not found: 3(NXDOMAIN)

No:

$ host XXXX.tplinkdns.com  54.87.217.253

Using domain server:
Name: 54.87.217.253
Address: 54.87.217.253#53

Aliases:

Host XXXX.tplinkdns.com.ynet.net not found: 4(NOTIMP)
Host XXXX.tplinkdns.com.ynet.net not found: 4(NOTIMP)