The fear of a zombie is not because it looks weird. It is because it is undead, literally a thing that should not be. Neither of those things are descriptions of tabaxi or dragonborn who are both living, breathing with thoughts and dreams.It is about Ravenloft and gothic horror as such.
You see, your vanilla FR players when encountering a zombie it is just another bag of hitpoints, to overdraw it a bit so you better get the meaning.
In Ravenloft your PC is expected to show (and best RP!) fear when encountering something supernatural, be it a Zombie or something like a Goblyn (yeah in Forlorn they are spelled like that, and they are a bit different to your normal goblins in so far, that their special attack is to bite you directly in your face)
So now you are expected to RP fear when encountering everything besides normal bears and wolves, and you come along wanting to play a Tabaxi or a Dragonborn? Darn, if I am to eventually run away from something looking like a goblin, even as an experienced and battle hardened adventurer, then what will normal townsfolk do, when encountering a dragonborn?
If you want to play Ravenloft like a vanilla campaign your free to do it but imho you miss out all the fun
Edit for more clarity: The need to RP fear and madness is even more essential to Ravenloft than what races you allow there as a DM, but deducting from that you just cannot allow races who fall more or less under the =monster-category. In some domains this might even include elves!