Welcome for your first post here.
And congratulations for using this great distro as Arch Linux.
Concerning your question, when the word 'warning' appears in a message, it is not an error but just something that could be better but is not crucial for the good functioning of the system.
So you can ignore those messages.
The 'xset: bad font path element' message is not an error neither.
It just means that you have not installed user fonts, above the system fonts.
I agree the message is not clear, but it's not more serious than that.
You can suppress the 'undefined mimetype/servicetype' messages (but it's not really important) either by creating the mimetype/servicetype in K Menu/ControlCenter/KDE Components/File Associations (I think it's there) or by commenting out the line 'MimeType=' in the desktop files mentioned in the .xsession-errors file.
But you can stay with these messages and others which may happen to appear in that file, if they are not critical (which is generally the case).
Enjoy your Arch Linux experience.