Why are oceanic ridges more elevated than the rest of the sea floor?

The mid-ocean ridge is a continuous mountain range formed from the uplift of thin crustal rocks from rising magma due to decompression melting of mantle material at divergent plate boundaries.

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Yes, that is how they are formed. They are high because, immediately below the ridge, the lithospheric mantle is almost nonexistent due to the heat of the rising mantle from the asthenosphere. The lack of cool mantle means that the relatively thin crust is the only thing "floating" on the mantle, and it is not very dense. As the lithosphere spreads (due to ridge-push force), the mantle below the lithosphere crust begins to cool and when it cools sufficiently it becomes, by definition, lithospheric mantle. This mantle is dense and grows thicker as the plate spreads further from the ridge and has more time to cool. The density of this, and the increased mass, is relatively higher than what is essentially only the crust at the ridge. Because it is more dense and "heavier," it sinks down further into the asthenosphere, causing the ridge itself to be high on the sea floor.