Is it normal to book a place at the park?

I mean if some one go there early then put the mat at the good spot then leave, go there later for Hanami.

Thanks.

Yes, it is normal, however some places have rules regarding this but they vary. Personally, I find it good manners when reserved spots are at least marked with the groups name and party time. That way the spot can still be used throughout the rest of the day.

It does vary. Some places allow the practice, others don't. some will allow you to lay down tarps but someone must stay with it and not just abandon it.

People are often too nice to say anything directly, but if the park is busy, they will be annoyed at seeing empty tarps lying around while they look for a spot.