There was a mixed reaction.

On the American side, this victory, especially when paired with the battles of Second Trenton and Princeton, was hailed as a genius move and praised as a revitalization of the Revolutionary Cause. Contrary to the popular imagination, Trenton wasn't Washington's first victory since New York, as he had beaten the British at the Battle of Harlem's Height. This was however these battles were the ones that really pegged Washington as being a really capable general for most people.

However, the battles themselves did not magically turn around the opinions of the soldiers in the ranks when it came to reenlistment. Washington had to really turn on the charisma and really work at getting the men to reenlist, as the soldiers hadn't been paid.

On the British side, it caught them by surprise. Washington's army had been in full retreat and they hadn't really expected he could launch a real counterstroke against them, even though they had always been aware that they would have troubles with American militia.

They were disgusted at how easily Washington had overrun the garrison at Trenton. The British higher ups laid the blame squarely on Colonel Rall, who was both the loser of the battle and no longer alive to defend himself and his actions. They wrote off his success as more of a fluke that had come about only because Rall hadn't taken all the precautions he could have.

However, when Washington successfully escaped the British trap at Second Trenton and thrashed the Princeton garrison, they grudgingly admitted that he had been quiet clever and gave him his due.

Sources:

The British are Coming: The War for America: Lexington to Princeton, 1775-1777 by Rick Atkinson

The Philadelphia Campaign: Brandywine and the Fall of Philadelphia by Thomas McQuire