ASCII input to HMAC weaker than raw bytes?
In one sense, no, encoding should not have an impact on security of HMAC.

On the other hand, it could have an application dependent impact.

Consider the following. '26' and 26 have the same HMAC. Now, assume your code receives a message, M, and an HMAC, MAC, and then does something like this

if HMAC(M, private_key) == MAC:
  if isinstance(M, basestring):
    grant_admin_privileges()
  else:
    grant_non_admin_privileges()
else:
  raise Exception('Bad HMAC')


Well, now obviously you are in trouble.