I’ve heard “méndigo” (used in Mexico) but I'm not sure if it's an appropriate word to use. Is it offensive, and if so, does it have this connotation in all countries?
The word "mendigo" without an accent is used commonly in Spain to refer to a person that asks for money in the street. It is a standard word and not inappropriate.
The word "méndigo", with an accent, meaning "infamous, very bad", seems to only be used in Mexico according to DLE. We do not use it in Spain.
I. 1. adj/sust. Mx, Gu, ES. Referido a persona, infame, muy mala y vil. pop + cult → espon ^ desp.
- sust/adj. Mx. Cosa despreciable. pop + cult → espon.
So it appears it is used with this despective connotation in Mexico, Guatemala, and El Salvador.