What I really love when I try Manjaro was the terminal. Few seconds ago I didn’t know that I can change the Terminal at all 
On the homeoage https://tabby.sh/ I find a Github link with .rpm files but also this
At the end the terminal told me “The repository is setup! You can now install package”
How?
And could I deinstall the stock Terminal?
It looks like the installer script not works fine.
Download the rpm and install it with dnf.
https://github.com/Eugeny/tabby/releases/download/v1.0.207/tabby-1.0.207-linux-x64.rpm
Then in terminal, cd into downloads and install it with:
sudo dnf install tabby-1.0.207-linux-x64.rpm
I would not do that … so you still have an alternative when tabby makes problems.
You might want tabby as your default terminal ?
Try this:
How to remove tabby:
sudo dnf remove tabby-terminal
My personal opinio:
I think it is a too heavy program with 381 M, the whole fuss you can get with alacrity as an example and configure as you need. Alacritty you get from Fedora as an rpm.