Don't do something that you know upsets another person, unless you have a compelling reason.
Anyway, back on topic, Raymond was clearly low-functioning by modern standards (but to the higher end of low-functioning). However, the movie was set before the translation of Hans Asperger's article and the 'discovery' of Asperger Syndrome, and back then, any autistic who used communicative speech was considered high functioning.
Regarding how HF/LF compares with severity of autism - I think they're correlated but not the same. I knew a sociable nonverbal 15 year old who used a lot of gestures to communicate and had obsessive interests. I'd call him mildly autistic, but low functioning. However I think severely autistic people pretty much always are medium to low functioning, even if they're quite intelligent, because their way of being so strongly clashes with our NT world.
There are many reasons why someone could be low functioning. Some of those are autism-related, some are just common comorbids. Things like mental retardation, motor apraxia, sensory issues, not being a verbal-sequential thinker, difficulty with symbols, each of those can contribute to your ability to care for yourself and/or communicate, which are the two key skills that define functioning level. Each can be seen in mild degrees among some/many high functioning people, and in severe degrees among many lower functioning people. But only some of those are part of autism severity.