Guy walks into a bar, wearing a nametag that says "Joe Smith". Bouncer checks his list that says "Don't let Joe Smith in!" and kicks him out.
Joe gets a pen and scrawls an "e" at the end of Smith, so that his nametag now says "Joe Smithe", and turns around and walks right back into the bar. Bouncer checks his list again and says "you're good, have a great time!"
MAC addresses are trivially easy to spoof. And since mac addresses are sent in the clear in 802.11 packets, it's trivially easy to sniff for a valid one.
With that said, if you're sure you want to implement mac filters on your IOS AP, here's how: