hey, i independently came up w the same idea, and i thought it might be nice to bump the thread. while you can add notes like this in pokepaste or other external sites, once the team is in your builder it's much easier to check there than to sift through logs with your friends to find that 3 month old paste they sent you. and ofc if you're ev'ing on your own you're probably not making pastes there really is nowhere more convenient to put this kind of info.
i think this is especially important on vgc and doubles where most pokemon have very specific ev spreads by virtue of games generally being shorter with less chip damage involved. i imagine this kinda stuff is even more important for them
if it's easier to code it might even be fine if the notes aren't in the UI too. you can just allow you to comment on the importable like you can do on most programming languages. that could work as a temporary fix if the teambuilder UI is too tricky to change for the time being?