Did Ragnarök myths influence the Revelation?
Nearly all of the imagery in the Revelation (singular; not Revelations) is borrowed from the Hebrew bible (i.e. the Old Testament). Of the remainder, most of it can be found in other first century Jewish and Christian literature, or in first century Greco-Roman myths and politics.
To break down specific points raised in the original question:
a concept [of 'Ragnarok', i.e. an eschaton] is not found in the Old Testament and is not associated with Jewish mythology and thought
Eschatological expectations of an idealized future, often after a period of conflict, are found in some of the later prophets of the Hebrew bible (e.g. Trito-Isaiah, Trito-Zechariah, Joel).
John's Revelation is a textbook example of a Jewish apocalypse. Early elements of the genre can be found in pre-exilic and exilic Hebrew prophetic literature. The genre really took off in the late Persian or early Greek era. The defining purpose of the apocalyptic genre is to reveal something hidden about the world, especially as related to the unseen spiritual realm and the future ('apocalypse' means 'revelation' or 'a revealing'). Elaborate symbolism is a usual feature in these apocalypses, such as Daniel, Enoch's Book of Parables, or Fourth Ezra.
The same period when apocalypticism was growing within Judaism, factions emerged in Judean politics. By the first century AD, the three most well-known factions were the Pharisees, Essenes, and Sadducees. Even at the time (e.g. by Josephus) they were defined by their eschatological views, showing just how important 'the eschaton' was in Israelite society. Similarities are drawn between the early Christians and both the Pharisees and the Essenes. To put it simply, eschatological beliefs were a major part of 'Jewish mythology and thought', and have a complex history going back centuries prior to the Revelation.
Similarities with Revelations/Ragnarok may be said to include the Sun being swallowed up,
The failing of the sun, moon, and stars is a common picture in ancient Israelite and Judean prophetic texts (e.g. Amos 5.18-20; Isa 13.10,13; Ezek 32.7-8; Joel 2.10). In the older examples, it is plainly idiomatic, denoting social upheaval, i.e. the disruption or overthrow of a nation. Later examples employ it in a more traditionally climactic sense, i.e. history-ending.
a great serpent rising from the sea,
This seems to be combining two different images in the Revelation, first the dragon which is thrown out of heaven (Rev 12.3-9) and is imprisoned in the abyss (20.1-3), and second the crimson beast which emerges from the sea (13.1) or abyss (11.7; 17.8).
The Revelation's dragon or sea-monster comes from the Combat Myth, an archetypal myth seen in many Indo-European cultures. In the ancient Near East this myth predates the Revelation by centuries, and can be seen across the Hebrew bible (e.g. Isa 27.1; 51.9; Psa 74.12-14; Job 26.12-13).
The Revelation's beast is an amalgam of Daniel 7.1-14's four beasts, each of which emerges from the sea. The Daniel 7 passage is vaguely reminiscent of the Combat Myth, but its specific verbage — one riding on the clouds, receiving an eternal kingdom — draws attention to a particular Ugaritic form of the Combat Myth in which Baal conquers the sea god Yam and receives an eternal kingdom.
the dead rising,
The resurrection of the dead can be found in at least three texts of the Hebrew bible.
The latest is the Book of Daniel, published circa 167-165 BC. It is almost certainly the final of the canonical prophets, and envisions 'the time of the end' as a period of crisis (the Maccabean Revolt) followed by the downfall of Israel's oppressor (Antiochus Epiphanes). Daniel 12 relays that those who died in the crisis will be raised to life for either reward or punishment.
Prior to this, the image of the dead being restored to life is found in Ezek 37.1-14, written circa 585 BC. Here the scene is widely agreed to be a metaphor for the restoration of the Judean nation after the Babylonian exile.
The third text is Isaiah 24-27. Dating for this text is disputed, ranging from the eighth century BC to the fifth or fourth century BC. There is also debate whether the image of the dead rising is meant to be a metaphor, as in Ezekiel, or literal, as in Daniel. It has been suggested the dead rising in Isaiah 24-27 has parallels in Ugaritic mythology (cf. Cho, Fu, 'Death and Feasting in the Isaiah Apocalypse (Isaiah 25.6-8)', Formation and Intertextuality in Isaiah 24-27 (ed. Hibbard, Kim)).
The Revelation draws on all three examples. The Revelation's structure is designed to follow both Daniel and Ezekiel, so that the resurrection is seen near the end of the book. Rev 20.4 καὶ ἔζησαν (they lived) is identical to LXX Ezek 37.10. And Rev 7.17 and 21.4 each verbally echo Isa 25.8.
fire raining down upon the world,
Fire as a form of divine judgment is replete across the Hebrew bible, and features heavily in apocalyptic literature.
and a horn sounding to signal the onset of the end.
In the Hebrew bible, trumpets are used in Israelite religious festivals, worship practices, and war. Of special interest here, the sounding of a trumpet is associated with eschatological expectations in Zechariah 9.14 (written in the sixth century BC) and Joel 2.1 (date disputed; third century BC at the latest).
Did the Ragnarök mythos influence the Revelation?
Almost certainly not.
The earliest evidence of the Ragnarök mythos, such as Thorwald's Cross or the Gosforth Cross, seems to be found no earlier than the tenth century AD. As the name of these objects indicates, however, these come from an age long after Christianity had swept across Europe. So though the Ragnarök mythos probably goes a ways earlier than the tenth century AD, we can't say for certain when it originated.
In contrast, a substantial chunk of the Revelation's imagery predate the book by centuries. The Revelation was written circa AD 90-95, and comes from a long-standing tradition of apocalyptic expectations within Judaism. Other than the specific Christian ideas present in the text and allusions to contemporary Greco-Roman symbolism (e.g. 'Babylon the great city' sitting on 'seven hills' corresponding to Rome 'the city of the seven hills'), most of the Revelation's imagery comes from Hebrew scriptures that are centuries old, which themselves contain pictures and ideas centuries older than that.