Yes it's true Uniswap reached that huge market cap and it is also right to say hype also contributed to that effect, how?, taking into account the exchange has been around for a while now as well as playing a huge role in the crypto space, thus when the token was released hype contributed in pushing it up. But can it continue to maintain that growth or will it drop? It's left for us to find out in the near future.
About its position as a top Decentralized exchange; I would say that, there is every possiblity that it will continue to maintain that position for a while this is because, in crypto exchanges, traders values liquidity a lot and since Uniswap has that, it will most likely continue beating others, until a new or old Dex comes up with another unique feature.