From the HHonors Terms & Conditions:
Participating hotel will determine whether incidental charges incurred during either of those stays are eligible for points, except that Points will not be earned for incidental charges at Hampton Inn®, Hampton Inn & Suites®, Homewood Suites by Hilton®, and Home2 Suites by Hilton®.
In my experience a vast majority of hotels will include a tip on a restaurant bill as an eligible incidental, but I find it difficult to argue that a hotel should not exclude that since they should be passing 100% of that amount to the employee and there really is no margin for them to be awarding points on it. I believe I have had at least one hotel where they did exclude tips, but don't recall specifically. In the end, the rules are pretty clear it's at the discretion of the hotel what is eligible and what is not.