I think if a owner of a forum or blog never or hardly replys to topics on their inactive forum then the forum won't be very fun to be on since most new members won't stay long if the forum isn't active and the owner does not try to reply to people's topics.
I agree with you. Posting a bunch of replies like "thanks for making the topic, thanks for being active, or some other generic reply" might make you look kind of lazy or posting for the sake of posting.