I personally LOVE that kind of attitude from players.
I don't want young players (at least on my team) in awe of the stars they're playing against that they grew up watching and emulating. I want them to go out there, and act like going against another nobody at the park, and try to kill them.
I don't care who I am facing, they're nobody to me. I can take them. They're not better than me. I can beat anybody.
That's the kind of attitude I like.
I gotta say I like it too, though not when it's directed at our guys.
I saw an interview with Mark Jackson awhile back where he said (paraphrased) "When I came into the league I honestly thought I was better than Magic, Isiah, Stockton, and all the other great PGs. I was dead wrong and had absolutely no right to think it but it was a huge benefit for me because it gave me the confidence to succeed at what I was capable of."
Irrational overconfidence is a much better quality in young players than being down on themselves, provided they don't take it too far. Jason Terry's a classic example of someone whose overconfidence helped him excel.