From the FontForge FAQ:
How do I mark a font as monospaced?
You don't. Just ensure that all the glyphs in the font have the same
width and then FontForge will automatically mark it as monospaced for
you. (If you mark it as monospaced incorrectly some rasterizers will
give strange results). If you want to set a font's panose values
yourself (Element → Font Info → OS/2 → Panose) then set the Proportion
field to Monospaced. This is necessary but not sufficient to mark the
font as a whole as monospaced. If you are unsure whether all the
glyphs in your font have the same advance width use Element → Find
Problems → Random → Check Advance.
This seems to work, so check it with your font file.