Sure, a good external GPS/galilkeo/glonass/… receiver with a good antenna, good software,… is mostly better than every mobile phone receiver. But it is always a hassle to get them, use them right, wait until they are locked down, sync them with the mobile… Also they do cost >50 €.
Over all, it is far easier and by far enough to use the mobile phone you already got, just start the app and all is fine. No need to sync files, no need to fiddle with lots of tools, hardware…
If you are addicted and dedicated enough, you can get a good receiver with good antennas, mount them on the outside of your car and sync them via bluetooth to your mobile phone and use mapillary with it.
But currently I cannot find any good gps receiver with bluetooth working out of the box and <100 €. And sorry, I do not want to fiddle with images and seperate gps tracks to combine them, if I can have the easy way. Not for a 2-5m better accuracy. Really.