Nevermind, I found a solution.
Renaming the .mcr files to .srm extension on my PC before putting them on my phone wasn’t working, but when I copied them to my phone’s hard-drive as .mcrs and made the change in the .Retroarch directory, it began to recognize them. I don’t know if this had something to do with administrator permissions on my PC, or on the phone, or none of the above, but at least the problem is solved.
So in the future, if anybody is ever searching for an answer, here’s what worked for me. Take the .mcr file from your EPCSX folder, put it in the .retroarch directory on your phone (using whatever your preferred method is), making sure all permissions are set properly. Then rename the file to the same name as the ISO in question, and change the extension from .mcr to .srm. Viola, it should work.