Is it okay to skip just the ceremony or just the reception as a guest?

If you RSVP for both you need to attend both personally I think it is very rude and disrespectful to the couple but if something really  bad happens you can excuse yourself to the bride and groom asap and maybe then you can other arrangements can be made if possible

As a guest who is invited to both the ceremony and the reception and RSVPs to attend both, then NO it is not acceptable to skip either. If a guest has a reasonable justification for not being able to attend either, they should let the bride and groom know in advance as soon as possible.