Any god or mythological being in Norse cult associated mainly with victory?
No, none that is exclusively associated with victory.
The closest we get is probably Odin, who is said to be able to give victory in war. Note however, the Sigrdrífumál, with accompanying prose introductions, in which it is told that Odin promised victory to the king Hjalmgunnar, but the valkyrie "Sigrdrífa" ('driver to victory'; she is actually Brynhilde) defeated him instead, so Odin's power is only worked through intermediaries. There is a part of Njal's saga in which a group of women weave witha gruesome set of tools: human skulls as loom-weigths, innards as the warp, etc, and sing of how they are valkyries and "weave victory".
If there would have been a norse personificiation of victory, the name would likely be Sigr.