How to change mob's Y coordinates spawn? Example: This mob spawn in overworld under the Y: 30 coordinates in underground

use natural spawns trigger ==> return==> your needed coordinate

Y 30 and below? abit hard but here, a sample, will definitely need more tweaking:

On the yelow box, set it to "cave air", also replacing the Y with y = +/-30 or just straight up +/-30 will only spawn your entity on +/-30, nothing else so that wont work, that's why I've added the "Can see the sky, in that way it's guaranteed to spawn with overhead, like caves. 2nd condition is cave air making the mob spawn spawn directly where a cave air is.

Also using the "spawn on xyx dx dy dz entity" then changing dy to +/-30 is pointless as dxdydz moves the entity after they spawn on xyz, so for ex, when a pig is spawn on the ground and you have it to dy= -30, the pig will be move downwards but any obstacle will stop it, like the grassblock the pig is standing at.

So yea, a lot of improvements is needed. 
Spawn, return, return are 100% needed. (Or else your living entity custom spawn condition settings wont detect the procedure"
Sample below makes a pig spawn underground lol