i'm in your conscience
^ Natures aren't totally important for in-game playthroughs. It's nice to have decent ones, but it's not a requirement.
Anyway, since I don't know your location in the game or how many badges you have, I can't say whether or not that's a good team for your current situation. I assume you're in Goldenrod since Mareep has Thunder. I do agree with Trainer Timothy though. I'd drop Butterfree and pick up an Abra where you can, or evolve the Eevee from Bill into an Espeon.
I'd also scrap Geodude for something else, just because that glaring x4 weakness to Grass and Water is a pain. Perhaps replace it with Lapras. I know that seems weird to replace a Rock/Ground type with Water/Ice, but Water is a great type and you really need one. If you're really partial to the Geodude, I recommend getting rid of Machop or Hoothoot for the Lapras. Hoothoot isn't too terrible, but I love owls so I am a bit biased. Plus I used one my first time playing SoulSilver and it wasn't too terrible. You could also consider Gengar. It's a great Pokemon.
Aside from team set up, you should probably work on leveling up for Whitney, assuming you haven't beaten her. Since you have a full team at the moment, I recommend your highest being 20 and you're lowest being 16-17. That's a good range for a full team imo. If you're at Bugsy, then your current levels are more than enough. And if you're at Morty, you probably want highest 24 and lowest at 20-21.