I think that it is ok if it is just for a few jumps, just "for fun". I actually have done it one or two times.
But then, if you are considering training "for real" in jumping, it would be better to buy a correct saddle. You don't have to buy a jumping saddle, but some saddles are multipurpose and allow you to jump and to flatwork.
I think that the main problem of jumping with a dressage saddle is that it is uncomfortable for the rider, and maybe for the horse too if you use it on really big jumps.
Just a bit uncomfortable for the rider but do check your pony for saddle sore.