No, don't add compost or other similar material to the height of the red line (red line is actually the place of your graft, the place where the rootstock and the scion meet).
By doing this, you will risk that your tree develops roots from the scion (the part above the graft), which would defy the purpose of having a graft. In addition, any change of soil ...
I think you have a case of a "reverting" weeping cherry.
This means as if you cherry wants to go back to its non-weeping form. This is not uncommon with weeping cherries. Here are some links describing similar cases:
Help, my cherry no longer weeps
Weeping cherry going straight
Weeping cherry tree reverting
If you don't do anything, the straight ...