Hello, maybe this is a silly question, but here it goes anyway 
I've been using ubuntu-mate for almost 6 years now and I'm very happy and I'm used to it.
I've got a new laptop with ubuntu 22.04 installed (the "standard" version) and I was wondering if it's recommended to install mate through tasksel or I should replace ubuntu with a clean ubuntu-mate installation.
Any advice is appreciated, THANKS 
I installed the Ubuntu MATE desktop Environment from inside Ubuntu. While it did work for the most part, I found there were many, many small issues as a result. I later then did a fresh install of Ubuntu MATE. All those problems disappeared. So I recommend a fresh install of Ubuntu MATE. Use the DE on Ubuntu to test out and see if you like Ubuntu MATE. I also would not uninstall the DE from Ubuntu as I heard that some dependent apps are uninstalled as well. Just my 2 cents.
Hello alete
If you use Debian you can install multiple desktop-environments and switch between them. I have done this myself.
From what I have read on the Internet over the years, this is not so straight forward with Ubuntu and it's flavours. I advise you to do a clean install of the Ubuntu flavour that you want to use. 
I would do a clean install. Default software like Caja and Pluma might conflict with Nautilus and gedit. There could be more files that have conflicts and give you grief, I just pointed those out as the obvious.
I'm an aussie, and until this month (Oct 2022) I've always had bandwidth quota restrictions on my home internet. My ISP also provides an official mirror of the Ubuntu repositories and they allowed that to be used quota-free, and until about a year ago official Ubuntu ISOs (but not flavors) could be downloaded quota-free. As a result; for as long as I've been using Ubuntu (since 10.10) my install was always with Ubuntu ISOs (ie. Ubuntu Desktop, Ubuntu Server), then I'd adjust to use the ISPs mirror so as to be quota-free, then add the desktop(s) I really wanted to use. This bandwidth quota issue at home introduced me to having multiple desktops, and I decided I loved it 
(for awhile I used to remove the default Ubuntu Desktop (GNOME2, then Unity) so as to get the desktop I really wanted, but then decided that wasn't necessary & it was often easier to have both installed; which allowed me to use the default GNOME2 which I loved, or Unity that didn't as much for me for a change)
If it was me, I'd keep your existing Ubuntu install, and just add ubuntu-mate-desktop. In the past I was more selective and introduced smaller packages, but I found that more work (and results could vary on release) so now just use the full meta package.
Do note: All installs I have have multiple desktops installed; it's my preference, partially as I find different DEs more efficient for some tasks & I can select which I'll use at login for a session, but mostly as switching DEs allows me to work on my machine and yet have the machine feel slightly differently depending on whatever mood I feel like when I login & start my session. Most of my DEs are setup so they're visually similar (MATE, Xfce, LXQt, very loosely GNOME) though my media keys do act differently depending on which DE is used; and I've not tried to synchronize that.
My primary PC died recently, it was dual boot with kinetic (what was then the current development release, only recently released) & a LTS (focal or 20.04), with Ubuntu, Xubuntu, Lubuntu & Ubuntu-MATE desktops installed on each. I'm using another PC currently (alas not even Ubuntu, but Debian bookworm) and it has ~20 session choices available on login; ie. more bloated than my Ubuntu system(s). I believe all my systems have multiple DEs installed.
With Ubuntu, I find having two [DEs] won't have problems, with issues only to be expected when you get to four (generally). I've written about it here if interested (where I talk about the my recent primary PC that has a dead PSU which I hope soon to replace).
Also note there are other costs with multiple DEs; you'll have more options in the (more crowded) menus, and selecting the wrong user-app can cause execution penalties if it uses different libs/toolkits to your currently running Desktop (ie. user-app & DE may not share resources), but my own knowledge means I realize this & consider this before starting the app. I actually still use devices with only 1GB of RAM, thus consider this automatically, but it's not newbie friendly! With MATE & GNOME there is far less conflict than if a Qt/GTK desktop/app choice was made.