the ath-m50x is easy to drive so no i would say you should be fine as far as powering them. most portables will even drive them well.
i have the ath-m50s which is the previous gen which is very similar minus the removable cables and they work fine on my desktop, laptop, amp, phone and my old phone.
the ath-m50 doesnt really benefit much from having a strong amplifier as it will sound similar to just using it on any normal power amplifier.
this leaves sound quality: if you notice any issues from using your onboard such as crackling, buzzing, ringing, distortion, low volume or other such issues then you might want to think about a soundcard to fix that. otherwise, onboard is fine
or, alternatively if you wanted virtual surround sound (for gaming) or access to an equalizer (without using one of the add-on 3rd party ones for windows itself) a soundcard can add those functions.
in short: not unless you find reason to need one. try onboard first.