If bombing places with seeds was a highly effective strategy for farming, reforestation and creating/recreating/restoring ecosystems, it would have been adopted as a practice decades ago.
Unfortunately, even on Earth, in environments conducive to the growth of forests, things are a whole lot more complex than this.
And another problem is the environment of the planet in question- there are different requirements for terraforming Mars than there would be for, say, Venus. You might be able to make a planet habitable by dropping things on it, but the question of how you're dropping those things, where you're dropping them, what exactly it is that you're dropping, what you expect it to do, and how you expect it to do it, are most important. And they are very big questions to ask.
On Mars, it might be possible to get CO2 ice on the polar caps to sublime, increasing atmospheric pressure and temperature on the surface, by dusting the caps with dark Martian regolith to reduce their albedo. This might be the first step toward a partially terraformed Mars- one with more hospitable temperatures and more survivable pressures- but a 'fully terraformed' Mars on which a functioning ecosystem can be grown would still be a fairly far away goal.