Is "nice guy" a euphemism for "ugly male"?

No. When I say "nice guy" I mean he's not a jerk. A lot of guys are. Whether or not he's a nice guy or a jerk has nothing to do with how attractive he is. I've known extremely attractive nice guys, and Ive known extremely jerky unattractive men. You may be thinking this because when we say a girl is "a nice girl" that's a nice way of us saying she's not attractive. Girls tend to sugar coat things.