A babies pee should not normally smell bad when it's fresh in the diaper.
If you're noticing that the baby's urine is starting to smell bad then you should have the baby tested for health issues such as a Urinary Tract Infection or other bladder infection.
If the baby's urine is smelling like fish then it could be a UTI or Urinary Tract Infection which sometimes clears up on it's own without much treatment.
But if the urine continues to smell it could be some other infection that needs antibiotics to treat.
Bacteria in the babies diaper area can also cause foul smelling urine in babies so make sure you keep the diaper area clean and use some baby powder during diaper changes to help keep the urine from coming into contact with the babies skin as much.
Make sure to change diapers often enough too so that the urine doesn't get into contact with the babies skin and cause bacterial infections.
I would highly recommend taking the baby to the doctor to be tested and let the doctor know about the bad smelling urine that the baby has.
Because bad smelling urine is not normal especially if it lasts longer than a day or two.