I have yet to try Mberry so I have no first hand accounts but I believe Mberry mostly alters things that taste sour - it makes things that taste sour taste sweet apparently?
I don't know what effect it would have on things that taste strongly chemically bitter. Chemicals like DXM are incredibly bitter in a chemical way so I don't know if MBerry would do much in that regard? It may interact with some of the syrup flavorings in strange ways but I don't know if it would alter the taste of the dxm.
I'd definitely be interested to try though. I'd be interested to see if it would have positive taste altering effects on things like DXM containing syrups, mescaline containing catcus brews, Poppy and Kratom tea, ayahuasca brews, etc...
Though again, most of these things taste horrible in a bitter way not a sour way, so I don't know if the Mberry effect would make them any more pleasant...
Edit: I did some more research after posting this and apparently MBerries can sort of mask/alter both sour and bitter things, thus making things that are either sour or bitter taste more pleasant. Perhaps it would actually help nasty things like DXM containing cough syrup taste better. I'd love to experiment and find out.