Its inspired by several things fans were talking about in various pony spaces regarding Izzy. Some people said she has "big mom energy" while others say she's a "milf in the making." Lol. I have no idea where its coming from. I think it has something to do with how objectively large she is compared to the other girl ponies. Another is people are calling her Pinkie Pie 2.0 and Pinkie was the only main character from gen 4 that got married and had a child in the finale. Note that this was before concept art of Izzy was released revealing her character was actually designated as "the caregiver" during the design process. Maybe there's actually credit to people seeing her as motherly. Me? I just saw a hippie.
As for Sprout, he's having a melt down because of a uni-mom attack. Izzy is his new mommy now.