Since it is a 'possible' stallion, I really don't see anything wrong with posting a pic and asking for a critique. At least that way, you can get some legitimate reasons to avoid the breeding.
Your mare is cute, but I agree with you, she's really nothing spectacular. Her neck is short and thick with a very thick throatlatch, her shoulder is a bit upright, it appears that she may be a bit back at the knee as well. BUT, the thing that turns me off instantly for breeding purposes is her front feet. They are very small for her body size and that can lead to a ton of lameness issues.