Thanks for your answer !
Unfortunately, the links you sent are exactly what I followed and tried to configure my controllers, but it didn't work.
The second link is for RetroArch emulators but I'm on mupen64plus (the only emulators that almost works on the RPI2).
Actually, all the modifications I made to the config files seems to have no effect.
And honestly, no my games are not running smoothly at all on the RPI2 😄 I have tried only a few games, some are almost ok but others are unplayable.
I'm planning to buy a RPI4 later but for now, I would like to be able to make everything work with the RPI2.