Cheesy is silly. It's when someone tries to do something for a laugh, but the other person will not find it funny. In fact, they think it's a little bit stupid. So if it's a smile, it's a goofy smile that doesn't impress. It's not cute. It's not funny. I am not even going to dignify it with a laugh. It's just silly. It's cheesy.

The adjective "cheesy" has negative slang meanings. If something is cheesy, then it is inferior or low quality in some way. A "cheesy smile" is a smile that other people perceive as being inappropriate for the social setting.

A childish smile. This article tells its derivation: https://www.urbandictionary.com/define.php?term=cheesy%20grin In addition to being childish, I'd say that it means a smile that does not look genuine.