no, I don't think so because you're just commenting on their tweet. Doesn't mean you're stalking them or something.
If it is disrespectful, you should do it, Celebrities (aka, the Twits on Twitter) deserve to be disrespected.
If you have access / allowed or public, to his Twitter My Space / Facebook / Blog page, then you may comment. He has the option to a) Block you, b) hold back comments till he realeses them, c) block all uninvited persons.
So go ahead, comment all you like...