It would only be rude if the elevator was full of people and you insisted in making a bunch pf people move out of the way just so you could push the button yourself. But assuming you aren't reaching over people or making them push back against others to get out of your way, a smile and a, "Thanks, I got it," is fine. I do think you should say thank you and acknowledge that the person offered to do something rather than just smiling at them though.