Is the melting point of pure indium too low to safely use to solder together a custom copper heatsink to be used ontop of motherboard mosfets?
It is too low if the MOSFET is going to dissipate enough power and there is enough thermal resistance from it to the heatsink (and ambient) for that power to result in a temperature rise that exceeds 157°C.
Without additional information, that's the best anyone can do.  Is it safe to use something that will melt at a certain temperature?  Sure, as long as it doesn't get that hot.
However, it is not good design to use MOSFETs close to their max junction temperature, and furthermore, most FR4 circuit board composite is only rated to 140°C, though some higher temperature versions are ok up to 175°C.  Regardless, on properly designed motherboard with sufficient heatsinking and cooling, it is very unlikely any MOSFET (or any other component for that matter) is going to ever get hot enough to melt indium solder.
That said, there is absolutely no reason to use indium solder.  It has a thermal conductivity of ~81\$\frac{W}{mK}\$ compared to SAC305 (standard lead free solder) which is about 60\$\frac{W}{mK}\$.  This is a largely moot point however the solder alloys with the metals it is joining and you can't really just look at the thermal conductivity of the solder at that point, but at the junction overall.
And the standard rule of thumb for the thermal resistance of two flat surfaces soldered together (with standard lead-free solder) is between 0.02\$\frac{°Ccm^2}{W}\$.  Even a tiny 3x3mm PSON MOSFET package has a thermal pad of 0.32\$cm^2\$, so the solder joint only contributes 0.0625 \$\frac{°C}{W}\$ worth of thermal resistance.  Using indium solder would lower that slightly most likely.
But this is a utterly negligable amount of the total thermal resistance.  Just the junction-to-case resistance of any MOSFET absolutely dwarfs this value, as does the heatsink to ambient.
There is no meaningful benefit to using indium solder for heatsink attachment to a MOSFET.   Just use regular solder.