There are no diminished returns on substats, but there are tiers and scaling differences.

For example, if a stat tier is every 50 points, You will broadly get the same benefit from 220 points in a stat as you would 249 points in a stat. You can easily see this with SkS stat, but it works for all of them. As someone told me, think of it this way: The game rounds down for stat tiers. Then as others like Barraind posted, certain stats scale better. So the more you have the better bang for your buck you get. A noted trend is that at the beginning of an xPac, DH is a better stat to meld for (at least for tanks and DPS generally - cant say for healers) simply because you wont reach the 'crit threshhold', or the point where Crit scales better than DH. That usually comes a bit later when gear has better natural values of it.

I recommend you go join the Balance Discord though. they got guides and what not for every class, as well as people who are on usually who can answer any questions and help you out. It's a bit more responsive than the forums. It's a good resource if your looking to work on your game play. Just be open minded and humble and people will generally be cool.