Gypsy Rose Blanchard says all of the letters Luigi Mangione is getting in jail will serve as a boost for the alleged murderer … but, be warned, she adds — ’cause there are lots of weirdos out there.

We caught up with Blanchard — who served seven years after she was convicted of second-degree murder in the death of her mother — at LAX Thursday … and, we asked her what it’s like to receive so much fan mail while locked up.

GRB says it’s good to buoy your spirits … but, it’s important to distinguish between people who are genuinely reaching out versus the ones who are sending letters in an attempt to get something out of you.

Just take her partner Ken Urker, Gypsy says … he sent a letter to her when she was locked up, and she could tell he was genuine. Now they have a kid together and couldn’t be happier.

Gypsy says she doesn’t have any advice about finding love behind bars for for Mangione — who is facing both state and federal charges for allegedly shooting UnitedHealthcare CEO Brian Thompson back in December.

Legions of fans have publicly expressed their support of LM … showing up in New York to cheer outside of courthouses where Mangione is going through court hearings.