The problem isn't so much that Ren lost, it's the way he lost.
Let's disregard the fact that he got clowned in the interrogation scene. Ren was winning the fight right up until he had Rey on the by the chasm. He had her where he wanted her, teethering on the edge of death and desperate, and he tried to seduce her.
Somehow, by mentioning the Force, Rey was able to close her eyes, concentrate and then pummel him in 30 seconds.
It's a shitty way to end a fight with a villain everything up until that point had been establshing as a real threat.
Ren could've recovered from being clowned in the interrogation scene, that's not so easily done from getting rekt in the forest. Episode VIII will have to do a lot re-establish him as a threat worth fearing.