Since 2018, there is a really lightweight, user friendly image viewer called qView that can view webp images. It also has a cool feature to resize the window frame to fit image. It is also free and open source.
You can download the latest version (3.0 at the time of writing, and it's still being actively developed) as a .deb file from the official website or github and install it with gdebi or your software center.
To install from terminal:
sudo add-apt-repository ppa:jurplel/qview
sudo apt update
sudo apt-get --install-suggests install qview
After you install the .deb, you also need to install this package: Qt 5 Image Format Plugins
sudo apt install qt5-image-formats-plugins
This is because qView is based on qt5 and even though it supports many popular formats by default, it does need this one package to actually view webp images.
Note: if you're using Lubuntu 20.04, this package is already pre-installed (and it might be pre-installed in other qt-based DE like Kubuntu, too - haven't tested).
Tested in Ubuntu 18.04, 20.04; Lubuntu 19.10, 20.04