No, it is not okay to do that. When you get invited to a wedding, it isn’t just to have fun but to watch your dearest take an important step. After the ceremony, the couple walks out through the aisle, giving them a bird’s eye view of those present. They will notice your absence and won’t think much of you if you just show up at the reception. So unless you have a valid reason which you’ve communicated to the couple before the wedding day, don’t skip the ceremony. If you do, don’t attend the reception, rather, mail them your gift.