What is the distribution mostly used by KDE developers?
I doubt there is a survey or statistic for this and it is also unlikely that people switched distribution when starting to work on KDE.
So most contributors will be on desktop distributions rather than server/enterprise distributions: Fedora, openSUSE, Kubuntu, KDE Neon, Arch rather than RHEL, SLED, Debian
I thought that most KDE developers used a distribution that had KDE as their main desktop environment.
It is unlikely that many are using distributions that don’t ship KDE at all, e.g. Mint.
The other distributions most often either have a KDE installer variant or make desktop a choice during installation.
Fedora, for example, treats Gnome and KDE plasma as equals, see https://fedoraproject.org/
People download the one they prefer.
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If you want to work on Plasma, it is simpler to use a distribution with the latest Qt versions or update it quickly and have a recent stack (wayland, systemd, pipewire, etc…).
That’s all rolling releases (Arch and derivatives), plus fedora, KDE-Linux (alpha), OpenSuse TW. I might forget some.
This will save you some time compiling Qt and other dependencies.
For application development (dolphin, ark, kate…), on older Qt version is ok. In that case most distribution are fine and you won’t have to compile Qt.
If Qt is too old (like debian, or slackware) you will need to spend time to compile Qt and potentially other dependencies…
That’s the main reason why personally I switched from Kubuntu to Arch, a couple of years ago.
That’s the reason why kubuntu can’t always have the lastest Plasma version too, because of the Qt version lagging for about six months every year.
So the choice might depend on your electricity bill and CPU power 
Is Manjaro a good choice?
In my experience, KDE developers prefer distros that update frequently. Some of the common ones in my observation have been Arch Linux, Fedora KDE, and OpenSUSE Tumbleweed. I’ve used all three and feel that each is a good development platform.
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Is Manjaro a good choice?
EndeavourOS or CatchyOs are also good Arch derivatives, each with their focus.
All of them will simplify the install over Arch.
Any of them would be a good fit.
You do you.
Fedora is going to plasma 6.5.1 in about 6 days which will have Qt 6.10.
I am mostly on Arch Linux. But sometimes on my custom images of distros. Eventually i want KDE Linux to become my daily driver but Flatpaks arent there yet.
I would recommend against it. It doesnt have a good track record and is sometimes even more unstable then Arch Testing. If you want an easy to use Arch there is nothing wrong with going to CachyOS.