I'm not sure if this is the official company stance, but employees in their forums actively discourage the use of LetsEncrypt http://www.inmotionhosting.com/support/community-support/general-information/using-freessl-specifically-from-httpsletsencryptorg
Also, the SSL that's included in their hosting plans is a SHARED SSL certificate, meaning in order to use your own domain name, you'll need to purchase a certificate through them or from a third party. More details here: http://www.inmotionhost