The meaning has really changed over the years, certainly since the ticket books went away.
Originally, popularity was the biggest factor. The higher the demand, the higher level ticket required. What was once an E, often is considered much lower now.
Nowadays, the scale is used more subjectively. It has come to signify the scale and scope of the attraction. Popularity isn't a factor.
WDI folks that communicate with me have, since day 1, put the Mine Train solidly in the D class. Mermaid is a C.
Nothing wrong with those ratings, the park needs rides of all levels. A D can be a great ride.