Disabling ligatures (see Calimo's
fonts.conf answer) is the wrong direction! It may remove those over-aliased "bold" ligatures in fonts like Calibri, but it also removes some of the beauty of typography. That's rather similar to shrinking the font until you can't tell the difference.
I solved this for my system by removing the Calibri font, installing Carlito, which is "metric-compatible with Calibri" and is packaged with "a mapping entry to fontconfig (local.conf)," and refreshing my font cache:
$ rm ~/.fonts/microsoft/CALIBRI*
$ sudo apt install fonts-crosextra-carlito
You can then verify that Carlito stands in for Calibri:
$ fc-match Calibri
Carlito-Regular.ttf: "Carlito" "Regular"
Before removing Microsoft's Calibri, I saved a test document in LibreOffice that used Calibri and took a screen shot. After doing that, I quit LibreOffice, opened it again, and then loaded my test document. The "Calibri" font name was italicized to indicate that it was substituted for. I took a screen shot of this substituted version and pasted it below the original:
The above text is a pair of screen shots of 13pt Calibri, with Microsoft's Calibri above Carlito as matched by fontconfig. There is no bold, no formatting, "pair kerning" enabled, as rendered by LibreOffice 18.104.22.168.
Carlito is quite close to Calibri, ligatures render properly, and it's much prettier overall.
You can do the same with Caladea for Cambria with
fonts-crosextra-caladea and you can use Croscore fonts Arimo, Tinos, and Cousine for Arial, Times New Roman, and Courier New with
This question asks about the Helvetica font. That's not a Windows font (Windows used Arial before it changed to Calibri). It's a Mac font. Here's my mapping (afaict, it's the default):
$ fc-match Helvetica
n019003l.pfb: "Nimbus Sans L" "Regular"
Helvetica has quite the culture surrounding it and I can't find a font that is "metric-compatible" with Helvetica. Liberation Sans & Nimbus Sans L appear to be the closest¹ among the free fonts.