Jalapeno peppers are much spicier than poblano peppers. This prevents them from being a good substitute for one another.  While jalapeno peppers have less heat than a serrano pepper or scotch bonnet peppers, fresh poblano is considered a mild pepper on the lower end of the Scoville scale. They are not as harmless as sweet bell peppers, but they don't bring the heat of many other kinds of peppers.