What do you all think?
It's good that you created it as open source, but I saw many similar websites already available, so you need to provide something special if you want to attract more people.You may want to add direct github link and open source license on the bottom of the page (without going to About page).
Even though it's a very simple idea I was quite shocked to find how hard it is to find a converter that is free of distracting ads and random information which I don't care much about (such as the price of other assets).
You can easily use Coingecko directly (since you are using their API) with uBlocker Origin extension, and you won't see any ads on their page 
I agree, it is easy to find tools like that. Even on coinmarketcap.com there is a nice converter
I know other useful websites that can convert Bitcoin to fiat currencies and smaller units, one of them was created by our forum member Bitmover if I remember correctly:
https://bitcoindata.science/bitcoin-units-converter.html
https://bitcoindata.science/bitcoin-units-converter.html
Thank for mentioning me.
This is a tool that allows to converte in basically any fiat currency. Additionally, you can also use all other bitcoin units, such as mbtc, finney, Satoshi and many others which are not very known