Covadonga is an heavily mythified event in the Christian side, with an importance blowned out of its historical proportions.
While there's no reason to think it never happened (after all it is considered by Muslim sources), a good part of what we know about it must be taken really cautiously.
It might be rather put in the context of a continuous harassing of Berber forces that were present in the region by highlanders trough a policy of fortification on several points, the battle itself being a glorified skirmish.
There's nothing pointing out that Christian holdings on northern Spain (rather than Asturian state, a collection of entities as Galicia, Asturias, Cantabrias, Pyrenean holdings, etc.) weren't already under the dominance, even if undirect, of the wali, as the whole region was as well as the rest of the peninsula, raided over in the 710's.
Pelayo basically seems (if we follow the whole account) to have refused to pay the tributes he payed before, implying he was under Arabo-Berber dominance as plenty of Christian entities in this case, not only in north-western highlands (kingship over Christians, the first name of this political entity, was built on the alliance of several small chiefdoms and the remains of the Duchy of Cantabria), but as well in Murcia, Septimania, in Lusitania, etc.
Now, what would have happened if Pelaio lost the battle, keeping in mind it was most probably a real minor battle there, if not a mere skirmish.
Basically, you broke the legitimacy of a local chieftain about imposing his hegemon on northern central Cantabrian Highlands. If he dies, I would likely see the Dukes of Cantabria having the lead there, as theythey were the main family in the region, and ended to takeover the Kingdom of Christians IOTL after the death of Favila.
The consequences, undubiously, would be limited : the main focus for Arabo-Berbers would remain Gaul (giving the revolt of Pelayo seems to have been tied up to the Arabo-Berber defeat in Toulouse, you just basically wait for this to happen again) and Cantabrian highlands would remain the god-forgotten hell-hole it was during Visigothic times until the unavoidable Berber revolt (which inderectly provoked a large abandonment of Berber military presence in the Douro basin) strikes on.
Consequences for the rest of Europe, at least up to very long term, are null. The situation in northern Spanish highlands litterally played no real role north of Pyrenees.
The most significant change you could maybe have, would be that the nasceant Cantabrian state would be more eastward leaning ITTL, a bit more reduced and delayed when it comes to its affirmation. But this much is already relatively speculative.