Another interpretation: [she was called Puah] because she would cry out [po'ah] and weep over her brother, as it is said, "And his sister stationed herself at a distance" (Exodus 2:4).
Puah Who Wept and Watched Over Her Brother
Yalkut Shimoni on Torah 164:6
דָּבָר אַחֵר, שֶּׁהָיְתָה פּוֹעָה וּבוֹכָה עַל אָחִיהָ, שֶׁנֶּאֱמַר (שמות ב, ד) "וַתֵּתַצַּב אֲחוֹתוֹ מֵרָחוֹק".