Hi Carol. While its possible, it could be tight. If you have the flexibility to move up your reservation a bit (say 5:15), you may wish to consider.
Here are some items to consider. First, while you may get seated on time, sometimes even with advance dining reservations you may need to wait a few minutes. I have eaten at
Mama Melrose's (very tasty) -- if you happen to order appetizers, meals and deserts, you could range from 45-60 minutes for the full dining experience. Lastly, this restaurant is located on the other side of Hollywood Studios and is a good 15 minute walk to the Fantasmic Show. So in best case, you will arrive on time or shortly before.
Since you are visiting at a pretty good time from a crowd perspective, you would likely find seats even if you arrive close to start time. However, as mentioned, if you have the flexibility to move up your reservation it may be worthwhile to consider. Hope this helps!