This is a problem with Final Fantasy in general. Every game has to end with a generic doomsday villain as the main threat, and saving the world from that generic doomsday villain is basically the plot.
Sometimes this works because the villain is memorable and/or charismatic and has a strong presence felt through out the story (Kefka, Sephiroth, Ardyn, Exdeath). Many times, it just doesn't work. Final Fantasy 13 was a disaster in storytelling, shown by its uninteresting villains who work with convoluted nonsensical plans to frankly stupid ends. Xande was such a non-presence in Final Fantasy III that hardly anyone even remembers him, largely being replaced by Cloud of Darkness, who is even more generic as it isn't even mentioned till the very end of the game and final battle, and is given next to no motivation aside from being pure evil. Nobody remembers that Zemus was the main villain of Final Fantasy IV; everybody probably thinks its Golbez. Ultimecia was as generic as one could get, with no real motivation or character established, no backstory given, and wasn't even seen till the end of her game. Final Fantasy IX barely avoids this by virtue of having Kuja upstage Garland towards the end. Final Fantasy X's villains are so uninspiring, that Dissidia made Jecht the villain representative for the game.
The only mainline Final Fantasy game that attempted to not do the whole generic doomsday villain schtick was Final Fantasy XII.