Simplenote is a free and open source app that you can use to take notes on Linux Mint.
Simplenote can be easily installed on Linux Mint, by downloading and installing its deb file.
Simplenote supports the most widely used Operating systems, which means you can run it on any device that runs : Linux, Windows, MacOS. You can also run it on your Android & IOS device.
In this tutorial, you will learn how to install Simplenote on Linux Mint

Step 1 - Download Simplenote Deb
Simplenote deb file is available for download on Simplenote's github repository. There are multiple ways to download the deb file. However, in this tutorial, we'll be using wget, a package that allows us to download files easily from the terminal.
Now, to download Simplenote deb file, simply run the command below
wget https://github.com/Automattic/simplenote-electron/releases/download/v2.21.0/Simplenote-linux-2.21.0-amd64.deb

Step 2 - Install Simplenote
The process of installing Simplenote's deb is same as installing a normal deb file on Linux Mint.
Launch terminal, and run the command below to install Simplenote
sudo apt install ./Simplenote-linux-2.21.0-amd64.deb
Once you execute the command above, you will be asked whether you want to continue. Press Y on this prompt, and then press enter as shown below

Step 3 - Launch Simplenote
Simplenote will be added on applications menu after installing it. To launch Simplenote via Applications Menu, you need to open Applications menu first, then go to "Accessories" category, and you should see Simplenote icon on the apps list on the right

And you will see Simplenote GUI as shown below
Simplenote requires you to have an account for writing notes. If you don't have an account at Simplenote, you can easily create one from the app, or web.

Once you login, you will see the Welcome window. Now you can start writing your notes on Simplenote.

Conclusion
Simplenote is a free and open source note-taking app built on Electron.
You can run Simplenote basically on any device, and this makes Simplenote one of the best note-taking apps that you can use.