Its not that much lack of shadow on the mob as that the mob simply doesn't belong on these maps. Maybe in a cave setting it would be more acceptable.
I played a server last year that had many new mobs thrown in without any mob sounds ('420' server?), not sure there were any shadows either, and it was playable. You had your own weapon sounds so it wasn't like silent movie. I thought it was worthwhile in that seeing new mobs compensated for the lack of sounds or shadows which you wouldn't tolerate if they were standard euro mobs.
What I find most grating in this regard is a bad mob dying/dead animation when the mob simply flops down (probably added rotated image of mob in photoshop so it looks flat, although sometimes it is so bad that it is funny). I've seen that on big servers too but you got to cut slack because it is a hard job to do all these things properly.
Red flame 'tiger' somewhat similar to the above was on Ben's server (I might be mistaken about the server owner) maybe couple yrs back? It was a sub or maybe a lord that spawned outdoor in BP and that was ridiculous on all counts.