The troll left his rude comment under a photo of Kelly and her kids ... saying, "[Matthew] could have his pick of women so just a little bit of makeup."

"If you weren't married you'd have it on."

Kelly was so pissed about the hurtful words ... she clapped back and called him "a typical misogynistic pig," while also sarcastically wishing him a happy holiday on her IG story.

But, just a few hours later, Kelly had a change of heart ... and shared her regret for stooping to the troll's level.

"I resorted to name calling which I truly do not like," Stafford said. "So I apologize to [the troll], but maybe you should think about if someone said that to your wife or daughters ... I'm sure you would be wildly upset."

Stafford continued ... saying makeup has never been her thing and her confidence doesn't even come from her appearance.

"It's a big reason I love my thirties because I am finally comfortable in my skin," Stafford said. "I know what I stand for and my values, and knowing who I am is what gives me confidence."

"Name-calling is not something I am fond of, so please accept my apology, but understand how upset you would be if the tables were turned."