You have to be top 10 to 20 to be extremely competitive and be plain jane.
If your mediocre and rub shoulders with the right people (heads of department) and prove them you are worthy. You can be just as competitive, as faculty and staff know your material wealth and how much of an asset you can be. You have to prove yourself.
People who want to specialize dont always get what they want and become general practitioners. This is by no means a bad thing.
I dont know your grades but lets say your middle of the road for arguments sake
Fundamentally, the beautiful thing about dentistry is that you can further your education in areas of specialty that you have interest in without having to specialize, so dont other people in your class discourage you. Just work your ass off is you want to specialize and if you dont work, your ass off so that your patients get the care they deserve.
In short, you will need A's, a few B's are ok, and a personality is optional

Also, a wicked board score does help make you look good on paper.