Interestingly I don't mind the Planzoo guests (that are the same models as Planco) but really dislike the look of the JWE guests. So although I appreciate you wanting the option to change it, selfishly I wouldn't want Frontier staff to put time into remodelling guests when they could be modelling new animals (and / or remodelling animals the community really wants changed). But that is personal opinion.
I don't know, how much work that would be. It would be a "nice to have", but I agree that new animals and building pieces and new foliage would be more important.
The guest aren't the most trouble with me, but the staff is (because I always can play without guests, but not without staff). And the staff sadly is even more cartoony than the guests are. I always want to

, when I see this silly jump the lazy keepers do, if they have nothing to do, or when I see this security guy nearlly falling over on his butt, because walking so much leaning backwards and resisting gravity.