No...this is not unusual in an enterprise wifi network. wireless is a shared half duplex medium as of today. In addition, using ARM, each AP will also be doing some minimal background scanning. Finally, ping is a low priority for the controller. Some drops are quite normal and expected. A better test is iPerf using TCP/UDP.
If it gets to be excessive, please see this thread for some guidance:
http://community.arubanetworks.com/t5/Unified-Wired-Wireless-Access/ping-drops/m-p/26762/highlight/true#M115