The plastrons as a whole are silly. They look like molded muscles and if you follow their various shapes down to they would seem to end on a weird point.
I've long theorized their various pants and loin cloths etc were a hastily added to cover up a problem a problem with how how their big bulky designs translated to live action ie they actors wouldn't be able to walk with full plastrons and shells so they just added pants rather than rethink their big hulking beast approach.
There's likely no in-story reason why the turtles have human-like faces. After all the maquettes all had more traditional faces for the characters so I doubt it was ever their intention. It's just a lazy, ill thought way of doing cgi.
If the human DNA thing is in the script it could make a nice fanon explanation although strangely wouldn't explain why Splinter who also presumably also has human DNA doesn't have a human-like face. It also strikes me that in the Fred Wolf show the mutagen originally worked to give the turtles human characteristics and even in things like Mirage book there's long been a fan question of whether TCRI's toxic waste is from the Utroms "skins" hence they would also have human (or close to it) DNA.
To be honest there's a certain charm to the turtles origin that it's shame to mess with but like most super-power origins doesn't really stand to much scrutiny hence the best super hero movies don't focus on them. What's more important than how they do what they do is why they do what they do but it seems that's being massively butchered too.
And many don't.
Baring in mind they seem to have toned down the actual noses it seems silly to me to stand by the corner that these are more realistic turtle designs when it comes to the faces when other aspects like their basic anatomy is so unrealistic.
