You can’t dismiss with prejudice because of a simple jurisdictional issue like that. “You should have filed it somewhere else, but now you can’t file it anywhere”. Prejudice means that the case has no merit and saying you should have filed it somewhere else indicates it is possibly meritorious.
I can’t comment on the use of “Uncle Tom” specifically.
But we ought not to use insults like that ever. By using them on people who “deserve it”, we are basically saying these terrible things are true, but we usually don’t say them because it’s rude. A better way to think is that we don’t use discriminatory insults/slurs because the hatred they represent is completely without merit.