YES! Cardio is very important for hearth health and conditioning. As a hard gainer, you wont want to do hours of cardio per day but I would do 2-3 session per week for around 20 minutes where you really get your heart rate up. It will only benefit you. Each one of those session will only burn around 200 calories.
Not only is cardio important for overall health, cardio is also great for stimulating appetite.
if you are saying run 30 miles a week, then no you will struggle to gain weight. when i was running 45 miles per week as an endurance athlete i was eating whole pizza's and still was 5% body fat
Cardio is important to heart health but this does not mean you have to add in cardio sessions. If you are lifting with low rest periods and keeping the heart rate up that is cardio as well.
I would definitely limit it if you are trying to add mass.