Our first cruise on Celebrity was last June on Summit and the next time we sailed was this January on Constellation. Between these two sail dates the quality of food has definitely declined. They are using lower cuts of meats for example. For example in the buffet (I know you asked about the MDR but this is a very clear example), many of the chicken dishes used chicken drumsticks, not even chicken thighs.
In the MDR they did not have prime rib in the 21 days we were onboard (a B2B).
So overall I was not thrilled however, for us, it was acceptable. The one other thing to consider is since they slashed the dinner buffet (lunch and breakfast are still ok) most people will eat in the MDR making things more crowded/hectic.