We're supposed to downvote if it is a bad answer, and incorrect one, or a badly written one. We can't really say it's incorrect, since it's a pretty subjective question (there are tons of suggestions you could have made, right? You only gave what you thought was best.) What you posted is good enough that you can read it. The grammar isn't perfect, but we're not grammar nazis here, and it doesn't really matter as long as you get the point across. Last one is the "bad answer." These are very subjective questions, so you're at the mercy of whoever answers it. It's not like "where can I find TM 35 in black" where there is a solid answer that can be proven. When you answer these questions, try to give as many suggestions as possible. If you give just thinly written answers, the reader isn't going to be as satisfied, ergo, voting you down for a "bad answer." I just summarized your answer here:
Victini-Give it flamethrower or Flare-burst.
Darkrai-Give him a Timid Nature.
Palkia-Give him a timid nature.
Rayquaza-Drop Overheat and use Dragon dance instead. get a Jolly Nature. (Actually, I think he had overheat to do a mixed emergency attacker, similar to Infernape, that's just a guess though.)
Groudon-Go with Jolly especially considering you already have Bulk-up for Attack.
Kyogre-TIMID, Water-spout, Thunder, Ice-beam, Thunderwave.
Overall team-It is a good team but I think you forgot that Earthquake damages your team mates, Groudon will take damage from Raquaza, and Kyogre will be 2-3KOed by Groudon.
The majority of this is just "change the pokemon's nature." That doesn't really help him out much, does it? you could have summarized it as:
I suggest going more with speed natures that way your pokemon could outspeed more dangerous threats"
In short, the answer doesn't really go deep into the team. Remember that the RMT is more subjective than the rest of the pokebase, and like the moveset questions, you're at the mercy of the reader. So don't answer a question unless it actually goes with several suggestions, possible threats, etc. Think from the viewpoint of the reader; they won't be very satisfied if your answer doesn't go very deep into answering their suggestion.