Is it normal that a molar crown leaves a gap?

Hi:What your dentist is telling you is true.  The shape and width of the implant is not the same as the natural tooth,Best,Dr. Maddahi

Hello,Implant crowns do sometimes have a tendency to trap food more often vs. natural teeth due to the diameter of the implant and the abutment being smaller than natural teeth.Dr. Mazaheri

That outcome is not ideal but at times happens.  There are limitations to the implants and many other factors that can affect the outcome.  We, as dentists, try to minimize those factors but sometimes just can't.
Just going by what you stated and not seeing a picture, x-ray, or knowing pre-op condition I probably would have said the same thing as your dentist did.

It depends on the type of implant you have. If a narrow implant was placed, it may be difficult to get the correct contours of the crown. If you have a broader, wider implant closer to the size of a natural molar, it is easier to achieve the desired contours to more closely simulate a natural tooth. We can get very close to a natural tooth but there's nothing quite like your own tooth. In either case, it's very important that you brush and floss daily to keep the implant clean.