Answer by taking a quote from the following article:

Maelgwn Gwynedd (Latin: Maglocunus; died c. 547) was king of Gwynedd during the early 6th century. Surviving records suggest he held a pre-eminent position among the Brythonic kings in Wales and their allies in the "Old North" along the Scottish coast. Maelgwn was a generous supporter of Christianity, funding the foundation of churches throughout Wales and even far beyond the bounds of his own kingdom. Nonetheless, his principal legacy today is the scathing account of his behavior recorded in De excidio

In his work On the Ruin and Conquest of Britain written c. 540, Gildas makes an allegorical condemnation of 5 British kings by likening them to the beasts of the Book of Revelation, 13-2: the lion, leopard, bear, and dragon, with the dragon supreme among them. He says that Maelgwn is the "dragon of the island", and goes on with a litany of moral accusations, in the process describing him almost as a regional high king over the other kings (the power-giving dragon of the Apocalypse). The Isle of Anglesey was the base of power of the kings of Gwynedd, so describing Maelgwn as the "dragon of the island" is appropriate.  Gildas restricts his attention to the kings of Gwynedd (Maelgwn), Dyfed (Vortiporius), Penllyn (probable, as its king Cuneglasus/Cynlas appears in royal genealogies associated with the region), Damnonia/Alt Clud (Constantine), and the unknown region associated with Caninus. The Welsh kingdoms are all associated with the conquest of the Gaels by Cunedda, while Alt Clud had a long and ongoing relationship with Gwynedd and its kings.  In the course of his condemnations, Gildas makes passing reference to the other beasts mentioned in the Apocalypse, such as the eagle, serpent, calf, and wolf. The reason for Gildas' disaffection for these individuals is unknown. He was selective in his choice of kings, as he had no comments concerning the kings of the other British kingdoms that were thriving at the time, such as Rheged, Gododdin, Elmet, Pengwern/Powys, or the kingdoms of modern-day southern England. That he chose only the kings associated with one king's pre-eminence (Maelgwn, the "dragon") suggests a reason other than his claim of moral outrage over personal depravity. Neither outrage nor a doctrinal dispute would seem to justify beginning the condemnation of the five kings with a personal attack against the mother of one of the kings, calling her an "unclean lioness".

Why did he make these condemnations?
The reason for Gildas' disaffection for these individuals is unknown.