Hi, @courrier 
(Usual disclaimer: please note that I'm just another Forum user here. I'm NOT an Ubuntu Developer and/or an Ubuntu MATE developer and/or a MATE Desktop Environment Developer).
You wrote:
That's very interesting! Thanks for pointing that out. In what version of "Ubuntu MATE" is that weird behavior happening in "MATE System Monitor"?
I don't know the reason for that behavior, but there's a somewhat related issue / bug that has been reported here in the "Ubuntu MATE Community", about the installation of software package updates showing the "UN" (United Nations) flag / emblem / logo. @ironfoot has guessed the likely "reason" / "cause" for that similar strange behavior / bug (related to the fact that "un" = "United Nations", but also "un" = "update notifier"). Here's the link to that answer by @ironfoot :
You (@courrier ) also wrote:
Indeed, you do need more Discourse rights to be able to embed images Meanwhile, thanks for having provided the Markdown source for the image that you tried to show. Thanks to that, I can show here that image:
In case it is useful, I suggest reading the following article that explains how you can get more rights / privileges / permissions in forums that are "powered" by "Discourse" (as you've correctly pointed out, this "Ubuntu MATE Community" is based on / powered by the open source "Discourse" forum software). In your case, I suggest that you read in particular the section titled "Trust Level 1 -- Basic" of the following article / web page:
I hope this helps