When I generally join forums, I tend not to really pay much attention to the activity of the admin when considered how much time to spend on it (if any, at all) so, I don't believe that it has a negative influence.
If an admin is posting a lot then, that's yet another person who is posting which in itself contributes to the overall activity of the forum which I can't see as being bad.
It's a given that staff members, especially the owner(s), to be the highest poster or among top 10 posters for example. I don't think it's a bad sign at all; at least it means he's really interested in posting and building a community. Plus in new forums people will be happier generally when staff members are actively responding to their posts and so on.
Turning things around it is not very necessary that staff members must be the highest posters. As long as they are consistent, numbers don't matter.
Its no surprise that the top 3 posters on my forums are the admins. However, I am the one who created the forum and have been there the longest but my co-admin's have out-posted me. 😛