After the final lines of ‘Mydwyf Merweryd’ (‘I am the Pulse …’) from The Book of Taliesin
Delightful to the dragon-lord
are songs from Gwion’s river
Flowing through the halls,
the scent of fair weather,
A horn full of mead
fragrant with honey and clover,
Druids skilled in Awen
– nothing pleases him better!
So the bard instructs the chieftain as to what is valuable and what, therefore, should please him: Gwion’s River (the flow of inspired song), fine weather, fragrant mead and the inspired utterances of his druids.
awenydd.cymru 
Within this context, could the dragon-lord be Maelgwn Gwenydd, or do you think Talieisin is referring to rulers in general?
There’s no indication that Maelgwn Gwynedd, or any other specific ruler, is addressed here. More that he is saying that rulers in general should listen to their bards! “Dragon’ is often used as a synonym for ruler or chieftain in this tradition. In the elegy for Llewelyn ap Gruffudd when he was killed and his head cut off, his court bard laments “A dragon’s head was upon him …”.
Though, of course, Gildas’ “Insularis Draco” is most pertinent here!