I won my first 6 placement games and then this happened...

If you fed, how do you not deserve to lose? Your jungler fed too isn't a good enough excuse.

I won my first 6 placement games and then this happened... I fed the enemy mid and top as jungle and went 0/10.

Depends, what did you do to try to make a comeback? Did you try to help snowball any of your team? Did you build to make yourself easier to carry? Did you play to your strongest team mates strengths?

Well tbh, if u won the first 6, that would mean the match making system put you into a higher MMR for the 7th match, in which you got dominated probably cause your skill level isn't...on par with the enemy team.