Robinson's men were beaten 21-15 at Pittodrie to slip to a third successive defeat since their victorious summer tour of the southern hemisphere.
The Scots saw off Tonga's south sea neighbours Fiji and Samoa in June but they were second best in their third and final November Test, with Robinson admitting his team weren't up to scratch at the breakdown.
"They dominated us at the tackle contest and if you get dominated at the tackle contest you end up being in a fight for the game," said Robinson, after Tonga picked up their maiden win over Scotland.
"It became a fight and Tonga scored a try and they were able to hold on.
"There are a couple of key areas for us. Our tackle technique and our tackle contest work is key: the core skills are vital.
"We've underperformed today and if we underperform we're going to get beat and that's what happened."
The visitors outscored the Scots by two tries to none, with Tukulua Lokotui and Fetu'u Vainikolo crossing for Tonga in the second half.
All Scotland could muster was five Greig Laidlaw penalties as they failed to build on a narrow 6-3 lead at the break.
Scotland won't be among the top eight seeds for next month's World Cup draw, meaning they face the prospect of a daunting pool at the 2015 tournament.