I know that some fans are emotionally attached to the TOS ship. That doesn't mean that when less invested viewers look at it they don't see a very old, outdated, simplistic design.
Look at this clip:
There is supposedly fantastically more modern Discovery next to the original Constitution. Here they both have the same crap level surface detail, so it is good for assessing the base shape only. And Connie stands up just fine. It is actually the Discovery which looks boxy and clunky here. Seriously, just give the Connie same sort of hyper detailed surface texture as the new ships have, and it will look absolutely fine, especially for a sort of short cameo appearance we're talking about here.