Chinese netizens use words sounding similar to their authentic ones to express many feelings.
肚纸 (zhi) is easier to pronounce thus childish (think of a child not able to say difficult syllable) than 肚子 (zi). We call this 卖萌/装嫩. Often you ll have emoticons and modal particles together serving that aim.
E.G. 咕，我肚纸饿了啦(／≧ω＼)，快点去吃饭吧( •̥́ ˍ •̀ू )
Likewise, there are 孩纸（孩子）/傻敷敷（傻乎乎）/靴靴（谢谢）/敲（超）……
But you might see various "substitute" of original phrases. Here list some of them.
"Transliteration" of English (not official or formal, but the character combination is fun)
and their further extension “我看你有如一池春水（sounds beautiful, hah）”, But actually 一池春水>春天绿色的池塘>碧池>bitch>he/she is cursing you.
Substitute by more "cute" homophonic characters