Mom replies with gibberish when asked questions. What kind of impairment is this?

Umm, we can only guess. Using words that approximate to the meaning of the word you can't retrieve when you want it is a common early sign of cognitive problems (nominal aphasia, I think, someone confirm?), but you really want to take her to a neurologist.

*Some* decline is normal, she's right; so reassure her that you would like the neurologist to confirm that this is the case and rule out potentially avoidable and/or treatable causes of anything more sinister. Why put up with a nuisance if you can do something about it AND possibly stop it getting worse?

I agree she needs to be evaluated by her primary physician, this kind of problem is called aphasia and it can have various causes

https://www.asha.org/PRPSpecificTopic.aspx?folderid=8589934663§ion=Causes
(from the American-Speech-Language-Hearing Association)

I'm not saying that your mom has Alzheimer's disease. But I will say that this is how my former mother-in-law, who does have Alzheimer's disease, talked eventually (that is, a few years into the disease process) while she could still talk.

I agree it sounds like time for Mom to have an evaluation. Explain to her that most health insurances require a yearly exam and so do doctors or they will drop you as their patient and she will have to start over with someone new.

Do not make mention of Alzheimer’s to her. No one ever said, “Oh boy! I’ll bet I have dementia!” But everyone in your family needs to know what the future holds.

When was the last time your mom saw her doctor?

I wouldn't ask her to got for this specific issue; I'd ask her to go for a regular checkup, get her flu shot, make sure that shes gotten any other vaccines/tests that her doctor might think necessary.

In advance of her appointment, send the doctor a brief, bullet-pointed note about what you are noticing.

She also says things like: there’s person who was sitting on the west, instead of on the left. When we asks her, do you mean, right or left? She just glazes over with no response.

when someone asks her, where is your new place? Instead of answering “it’s just a mile west of the airport”, she gestures wildly with both hands trying to draw an imaginary map in the air, but of course nobody understands what she’s trying to do.