Voicing for a cartoon isn't ADR. That's just voice acting, and if you stick any actor alone with a tape recorder chances are you're not going to get good results since there's no director and no other actors to play off.
In ADR you are listening to playback of the scene you performed on the set and trying to match it, so sure, it's possible Nimoy couldn't recreate his on-set performance, or maybe the director in ADR session asked for something different or picked bad takes.
The most noticeable Trek ADR to me is in TWOK when Khan says, "You mean he never told you the tale? To amuse your Captain, no?
" where the sound quality is utterly different on the italicized text. It sound like Khan is suddenly speaking over a telephone line.