I mean, it's ambitious enough to be a D Ticket, even if it's not the best one. The Skyway was a D Ticket.
Considering the Swiss Family Robinson Treehouse and Main Street Cinema were B Tickets, it's certainly at least a C Ticket. Dumbo and the Mad Tea party were C Tickets, and there's no way Mermaid ranks lower than them, even accounting for personal tastes.
I'd say Mermaid and Seven Dwarfs Mine Train are squarely D Tickets - bigger deals than Winnie the Pooh or Peter Pan's Flight (if somehow also less satisfying), but not running up with the big dogs like Splash Mountain and Rise of the Resistance. You can tell when they've swung for the fences, and nothing in New Fantasyland fits that bill.
Which is fine, since Fantasyland needed capacity more than it needed headliners, but they somehow fumbled it and we've ended up with hour+ waits at Mine Train. They correctly identified the problem of Fantasyland being the place where "the smallest guests wait in the longest lines for the shortest rides", but the execution failed to actually solve those issues.