No. mobi's have been supported since day 1 on the Kobo eReader devices. HOWEVER; mobi support is not what one would call robust.
If you have much content in mobi format you would be advised to use calibre to convert to ePub format for Kobo devices.
PS: I'm requesting this thread be moved to the main Kobo forum; this subforum is for the Android series of devices (Arc / Vox) and not the eInk (Glo / Touch / Aura / ...) devices.
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