Most resources I would think are not actually found on the Methane covered surfaces.
If we take our own system for an example. Would the Hydrans want our planet? No. They can't live on it, and while theres probably a lot of minerals still yet to be found, you'd have to dig pretty deep. Everything that is easily accessible has already been mined, and used, over and over again.
However, at the present moment there is untold amounts of raw resources in the Asteroid Belt, on Mars, Venus, Mercury, Jupiter's Moons, etc. If they wanted them, they'd have to kill us, pacify us, or drive us off, because if they didn't we'd fight for our resources, harassing their freighters, and what not.
So while the Hydran Planets themselves may not be worth much to the Klingons, that doesn't mean their star systems are worthless.