???
I have a juniper wher the foliage is all grafted on.
Grafting is a common bonsai technique.
Now, some grafts are better than others. If you're talking about the grafts on trees we commonly see at landscape nurseries, those are often poor grafts. For bonsai. Those trees will not be under the scrutiny that bonsai are, so the graft makes no difference. But in a bonsai, an ugly graft is a major flaw!