I think the problem is the attempt to appeal to our sympathy.
BioWare chose to do this by making the Catalyst appear as a child, which was unsuccessful with me. You're suggesting to do this by making the Catalyst appear as the VS, which would be unsuccessful for different reasons.
Either way, an effort is being made to disarm me by putting a face on the Catalyst which is innocent or familiar. Both are probably just going to put me more on my guard, and both play with big uncertainties -- the uncertainty of whether the player actually cares about the child, or the VS.
The solution? Eh, fwiw, I think the Catalyst's problems went deeper than the mere aesthetic, so ultimately I don't think I would have liked it no matter what face it wore.
But... if I had to think of something to avoid, it might be going for the Someone You Care AboutTM avatar. It may have helped if it had appeared as someone or something neutral, or perhaps if it had cycled between a range of fallen squadmates or characters. Then at least you'll achieve the sum meaning of all those losses, it doesn't feel quite as much like targeted advertising, and there's a greater chance of hitting someone the player may have given half a crap about. And even if it doesn't hit anyone the player cares about, it's a cool effect to see all the fallen return as the Catalyst's puppets, as the list of casualties will be unique to your playthrough and choices. Mind you, I'm no developer, and it may well have been impossible -- or at least incredibly impractical -- to implement that effect.