Here is a list of all of the times that I could find where the word "lowly" appears in the official English translation of TGCF. Please excuse any mistakes. I had to sort through a whole bunch of the word "slowly" and it doesn't sound like a word anymore.
Xie Lian replied innocently, “It wasn’t us. Truth be told, I’m but a lowly scrap collector.” Vol 1, ch 6
[Hua Cheng, about Qi Rong] “You mean that lowly, vulgar trash?” Vol 1, ch 11
Xie Lian rubbed his forehead. “Xianle is but a lowly scrap immortal, without power.” Vol 2, ch 12
The State Preceptor wiped sweat from his face and smiled. “Splendid indeed. But unfortunately, this lowly subject didn’t come up with it." Vol 2, ch 25
[Mei Nianqing] “It is the fortune accumulated from three lifetimes that gave this lowly subject the chance to become his teacher.” Vol 2, ch 25
[Qi Rong, to Feng Xin, in all caps, made plain text for accessibility] “You damned audacious lowly servant, you dare hurt me?!" Vol 2, ch 26
[Qi Rong, to Mu Qing] “And you! You lowly servant.” Vol 2, ch 26
[A ghost, to Yin Yu] “This lowly one won’t ever dare to sell counterfeits in ol’ lord Hua-chengzhu’s territory again!” Vol 4, ch 69
[A Qi Rong minion to a cultivator] “We lowly ones were only doing this for a bite to eat!” Vol 5, ch 74
[Narrator (arguably Xie Lian POV)] If the esteemed Martial God of the North couldn’t even defeat a lowly ghost blade on the outer edges of Mount Tonglu, how could he live with himself? Vol 5, ch 78
[Narrator (arguably Xie Lian POV), about amnesia'd Xie Lian] No one wanted to hear others say they were a lowly collector of junk. Vol 8, ch 137
That's all! Nothing in volumes 3, 6, or 7
All my support to fic writers who make whatever choices they want, including following fanon, but I had to satisfy my own curiosity as to whether Hua Cheng ever self-describes as "lowly" in the canon (translated) text! It appears that he does not (though Xie Lian does). He does, however, call himself a bunch of other fun things at least hypothetically, like "the bones you need to trample on your climb" (vol 8, ch 130), and he declines to argue when Mu Qing calls him "disgusting" (vol 6, ch 93)!