Left eye sunken after cataract surgery. Is it due to the steroid drops?

It isn't likely to be from steroid eye drops. Sometimes this can happen with steroid injections under the skin of the eye.
More commonly, it happens when there is more inflammation than the eye wants and the lid naturally droops to help with light sensitivity. In these cases, its because their isn't enough steroid. With time or more control of the inflammation the lid goes back up.
If a stitch was used it may be irritating the lid from the inside and a droop can occur.-
Not of these things you can self diagnose. See your surgeon.

It is possible that your eyelid became droopy called ptosis.  Ask your surgeon.  It is unlikely to be caused by steroid.