It's actually just the opposite. If you're not realistic then you're not really an optimist. True optimism is having a decent knowledge of the facts not just blindly believing in something.
With regards to BYU, they are still a work in progress under this coaching staff. That defensive performance would have led to a loss against most top 25 teams (Houston is decent but they are more likely in the 40-55 range). Those of us, myself included, that were worried about playing Houston based on the 2nd half defensive against a very average UTSA offense were exactly right to be worried. BYU should have been ready to switch schemes a lot earlier and there never should have been a 23-0 run by Houston.