No. I think MondoMega is on the 4 echoes train cause it would make a perfect character select screen? (I can't remember lol)
Nah, there's plenty of reason to think that we've got four more characters of some sort in the tank. Mostly because of datamining.
There are placeholder slots for fighters in two places in Smash Ultimate: the fighter directory, and the world of light directory. Both point towards another 10 fighters total.
The fighter directory had 30 placeholder slots when the game launched, all of which crashed the game when you tried to force them to load. As Piranha Plant and Joker got added, they took up two of these slots, and they no longer crashed the game. However, during Hero's update, not only did his slot get changed, so did the next 13 after him. At that time, slots 4 through 16 now all were set up to load Mario as a fail safe instead of crashing the game. Since then, Banjo-Kazooie, Terry, and Byleth have taken up three of those slots, leaving us slots 7 thru 16 still set up with Mario failsafes, whereas 17-30 still crash the game. These slots have been set up to be able to host fighters properly, so I'm of the opinion that they'll all get used before Smash Ultimate is complete.
World of Light's slots are weird though. There were also 16 of them there, but the files have been weird since Banjo-Kazooie's update. For Plant through Hero, they replaced the existing unused slots, much like they did for the Fighter directory. However, Banjo, Terry, and Byleth's files were added on after the 16 slots, and didn't replace any of them. Making things weirder, one of those slots was outright deleted during the Terry update. Terry didn't replace this slot, though - his WoL file was still placed after all the other slots. So currently, there are 12 unused WoL slots, with one having been nixed during Terry's addition. Given the weirdness here, I'm not sure if we can count on this one as much. The fact that, through Hero, there were also 16 slots is noteworthy though, even if things got messy post-BK.