Okay, this may get a little wierd....
Tien IS strong, by Earth standards. If all androids, Saiyans, and other life forms were excluded, I think Tien may possibly be the strongest fighter. Now, add in all those who we just took out, and Tien becomes a second rate fighter.
Now, in reference to Cell.... Cell was suprised. Cell KNOWS the strengths of each Z fighter, so he wasn't expecting Tien to up him. Tien's strength came from anger, hatred, and a will to help his friends. he knew he was their last hope, so he put EVERYTHING into his attack. That is the only reason he was able to hold off Cell for that time.