It wasn't Jenova. You can attribute moral responsibility for Sephiroth's actions to Jenova to a degree, but Sephiroth was the one that actually carried out the plans. There's no way around that. Not only that, it's not like Jenova was possessing him or something; he cooperated with her willingly.
Sephiroth did quite a lot in terms of his power. He fully expected to not only handily survive the meteor, which appeared to be the size of Texas or larger, but actually absorb all of its energy and the Lifestream, to boot. This is a much larger energy release than the Chixulub impact that Chrono Trigger attributes to Lavos. That one was "only" about 100 teratons; the nickel-iron asteroid involved in that was only 5-10 km in diameter. An asteroid the size of Texas would do exponentially more damage, at around 900 km in diameter. To give an idea of scale, that would basically equate to taking six to ten complete Death Star II superstructures, binding them together with uber-superglue, and smashing them all into a planet (the six to ten figure is used because most of what isn't taken up by the reactor is hollow). Your characters can't equip materia that powerful until the end of the game when you get the Master Materias, and even then, basically anything but King Arthur's spectral crew would be helpless to stop the incoming meteor. Kefka couldn't have done that in a million years. Kefka used the statues to rearrange the face of the world, not his own power. Even after becoming ruler of the world, the best he could do was blow up a town every once in a while. Not only that, he almost certainly had a higher death toll than Kefka beforehand; he did bust into the Shinra Building, decimate most or all of the guards and other personnel, and kill President Shinra. Considering that Shinra basically is the government, that's no small feat.
