Is it permissible for a pilgrim performing hajj to go directly to Arafat on 8th Dhul Hijjah instead of Mina due to the heavy crowding that occurs when ascending to Arafat?

It is permissible for a pilgrim to go directly to Arafat on 8th Dhul Hijjah, i.e., the Day of Tarwiyyah, and there is no harm in this as going to Mina on that day is a Sunnah.

The eighth day of Dhul Hijjah is called the Day of Tarwiyyah because in the past the pilgrims would rest in Mina on this day. They would also rest their sacrificial animals (hady) and give them water on their way to Arafat in preparation of the rituals of this great day and the rituals of the Day of Sacrifice and the Days of Tashreeq.

It is recommended, though not obligatory, for pilgrims to go to Mina in mid-morning and perform the noon, midafternoon, sunset, and nightfall prayers there, shortening the four-rak’ah prayers without combining any of them. The pilgrims are to spend the night before the Day of Arafat in Mina and perform the dawn prayer there before proceeding to Arafat in mid-morning. If a pilgrim does not follow this order and instead goes directly to Arafat on the eighth day of Dhul Hijjah out of concern for overcrowding, they are not blameworthy and their hajj remains valid. In doing so, they are merely omitting a recommended act for a valid excuse. It is hoped that they will receive the reward for what they would have performed had they not been prevented by that excuse. A compensation (in the form of sacrificing an animal) is only required for omitting an obligatory act of hajj, not for a Sunnah. 

And Allah Almighty knows best