I don't know about the exact origin. I just know that "heck" and "frick" was used a lot on tumblr half-ironically, kind of like owo (and its related copypasta). Ever since tumblr banned porn a lot of its associated subcultures and lingo just spread to other social media like twitter and reddit.
The heckin cute and valid enbie sort of represents a lot of the culture and memes in nonbinary and xenogender space on tumblr and the places they spread to. if you look in some of the spaces you'll see a lot of "you are valid" type posts or memes. memes also dunking on "truscum" (trans people who believed you needed dysphoria to be trans) happened a lot because xenogender spaces viewed gender as a social construct. "Heckin" wasn't used as heavily but it wasn't out of place.
I also know the abuse of "heck" is also used a lot in the abuse of doggo speak ("you are doing me a heckin frighten") but i think that's more of a reddit thing. i think there's overlap though, with drawings of tiny animals using "heck" instead of swearing to seem intimidating.
I think the "hecking cute" and "hecking wholesome" makes fun of the doggo speak type redditors more but grouping them together isn't that uncommon.
It's possible someone can give you a better answer OP, a lot of this is from personal experience and wasting my teen years being chronically online.