Dictionaries give two distinct meanings to the word "mentality" the origin of which is an English derivation dating from 1690 that combines "mental" with the abstract noun forming suffix -ity, which indicates a state or condition. The first meaning is mental power, endowment, or capability. The second meaning is one's mode of thought, one's turn or cast of mind, how one's view or outlook is set. In relation to The Flower Girl in Act 1 of Pygmalion, the second meaning promises to be the more interesting, though information provides enlightenment on the first meaning as well.
The Flower Girl, according to the first meaning of natural mental endowment and capabilities, is shown to be rational and intelligent. Though spoken in an almost impossible to decipher dialect, her remark to the Mother is telling of significant mental endowment and capability. She says (in translation),
"Oh, he's your son, is he? Well, if you would have done your duty by him as a mother should, he would known better than to spoil a poor girl's flowers and then run away without paying."
This speech includes a tag question ("your son, is he?") which indicates (1) a question posed within a statement and (2) recognition of an ironic situation and (3) an opportunity for recouping her losses.
Her long subordinated "if/then" statement ("if you ..., he would ... paying.") is a third conditional that describes an unreal situation from the past that has impossible consequences in the future or present: If you had taught him, which you didn't, then he would be good, which he isn't, and he would not have run off, which he did. The same sentence also indicates complex cause and effect, showing two effects from one cause ("if you ..."/cause; "he would know ..."/effect; "without paying"/effect). It also shows complex organization of chronological sequence that combines past ("you had"), future ("he would"), and present ("without paying") in the same complex third conditional.
Finally, she follows this persuasive conditional argument up with a direct appeal, which is half appeal and half challenge, to recoup her losses (in translation): "Will you pay me for them?" The conclusion is that The Flower Girl's mentality (meaning one) shows an acute endowment and more than ample capability: in other words, she's very intelligent.
The Flower Girl, according to the second meaning of turn of mind and set, or cast, of one's views and outlook, is shown to have a high moral character and regard for the law, about which she is knowledgeable, as well as an acute fearfulness owing to society's strictures on poor persons even appearing to step out of a respectfully subservient role.
First, her moral character is demonstrated in her truthful insistence that she is "a good girl" and in her rightful reprimand of Freddy's mother for his callous disregard of moral and legal right. Next, her high regard for the law is demonstrated in her repeated citations of legal principles pertaining to her rights as a flower seller, for example, not sell on the "kerb," not talk to gentlemen except to sell flowers. Her fearfulness of society's strictures, legal and cultural, is demonstrated by her wailings and moanings, which are caused by the thought that a "copper's nark" might be taking her "words down." This further points out her natural and keen sense human dignity as she asserts her dignity in claiming her words as her own.