If Duo had constructed the example the way you suggest then students might not notice that in this sentence it is the object that is feminine not the subject.
The gender of the subject is not specified which in French means it has to be taken as masculine. Masculine is assumed unless indicated otherwise.
This particular sentence introduces three of four concepts at once that beginning students find tricky.
"c'est une personne froide" can translate in: - he is a cold person (re. rule of he/she + is + noun -> c'est un/une) - she is a cold person (same as above) - it is a cold person - this/that is a cold person.
In this sentence, "une personne froide" is not an object, but an attribute (after verb être, paraître, sembler, devenir, demeurer, rester, all being "state" verbs, the noun qualifies the subject, so it is not an object but an attribute).
Same in English.
If your English is clumsy because you are learning the language and people see you as making mistakes, then she is hot will generate a snicker from some people. Even when it is clear that the person is hot because they are sweating and complaining etc. some people will laugh.
Someone teaching English as a second language might caution students to be careful using such phrases especially around students. Sensitive students hearing this warning might draw the conclusion....never say she is hot .........or similar, because the embarrassment would be extreme.
Of course, a native English speaker would have no hesitation describing someone as hot or cold or good, if that is what they were.
If you don't want to use "un bébé", you can say "un nouveau-né" but you still don't know if the baby is a girl or boy. If you don't want to use "une personne", you can use "un individu", but you still don't know his/her gender. Same story with "un témoin" (a witness), "une victime" (victim) and a few others.
This is not a vast difference, since in English you don't know anyway.