Kobo's firmware only allows the user to increase the font weight of native fonts. It is possible to change the software to activate unsupported features, including the option to change font weight of side-loaded fonts, but this requires patching (see here for further details).
I prefer to avoid altering or interfering with the native firmware unless there is a necessity, e.g. installing KOReader for better PDF support. However, similar to activating full-screen reading and exporting annotations it is possible to change font weight without patching. To set the font weight the following code needs to be added to eReader.conf (the file is located in .kobo\Kobo) in the [Reading] section:
The code above changes the weight of 'Bookerly' to 0.20 but the code can be changed to work with any sideloaded font. The weight can also be changed and it might be a good idea to test different weights before identifying what works best. It is also recommended to open and edit the file with Notepad++ to locate the right section to add the code.