Sorry for my ignorance,but I have a horse that is slightly downhill...His hind is just a tad higher than his withers...

Is this a problem?

Not unusual if he is young, they tend to catch up.

As wymple says it depends a lot on how old he is.
Lots of horses don't level out till they are 4 to 5 years old, I know gelding that grew until he was seven
What you can do though is to take a stringand measure him from the back of his elbow to the center of his fetlock, and then flip that distance back up again from his elbow and that will tell you very closely how tall he will be when he stops grwowing.
If the distance from his elbow to the ergot on the back of his fetlock is the same distance as from his elbow to the top of his whithers he is done growning.

Is a downhill built horse a problem? I think tha it is, I have never owned one yet that did not breakdown in the front end.
But I was roping intensely on them, if you were just riding them casually perhaps it would not be.
Either way in my eyes it is a serious physical fault, and highly prone to leg problems.

It is sure as hell harder for them to stop hard as in a reiner or cutter or heeler or calf roper.