Is the skim coating on my ceiling ok?
Well, this is thicker than a skim coat, but it's normal.  The compound will contract somewhat as it dries.  The normal and correct procedure here calls for multiple coats.  It is time consuming, but the only way to do it.  At any rate, inconsistencies will certainly exist between coats.  Its the final coat and finish that is important.  If this were supposedly a finished job, that would be a problem.  But this is just a work in progress and I wouldn't worry.  The only possible concern would be in the first pic.  That kind of does look like a crack which may indicate poor adhesion.  But it is hard to see.  The second pic looks like normal shrinkage of a somewhat thick coat.