Polkadot is multichain network, meaning it can process many transactions on several chains in parallel. Dogecoin is a meme coin created like a joke, built on the Bitcoin networks. There are huge usercases of polkadot platform unlike the dogecoin, it is a meme coin use for tipping on certain community of users.
Currently, dogecoin has large community of supporters and this help to boost it's price, while plkadot can boast of a good community with excellent usercases that has also promoted the coin.
I think that coinmarketcap ranking is not a constant position, any good projects can spring up a surprise to becoming a top coin/token at any time.