I don't find that there are any uses for lasers that are particular to MK, but you use lasers against MK in the same ways as you do against most characters.
Which is mainly just zoning them out, or in supporting your approaches at mid-range if you need to approach for whatever reason
However since nado exists, if an MK can predict you using nB (or sideB) or just reacts early enough (especially to a charged nB) then he can nado to punish you. So don't make it a habit.
All in all I guess it's less useful against MK than most chars.
Also I hope to see the other ZSS mains start using laser locks on stages with platforms...it's so easy, you just edgeslip the opponent by hitting them off the platform (probably with bair/fair) and then laser them when they hit the ground and it forces a reset.
If you time the followup dsmashes correctly (not hard) you can hit them with 1 dsmash after the laser but before they start the get-up animation, but they won't get stunned from it. Then 2-3 more dsmashes after the get-up animation. It's a lot of damage lol.