It's offensive because mild is how you perceive it, not how we feel it.
We can have lower support needs, and that may remain so throughout our lives, or may fluctuate, there will also always be people with higher support needs all their lives.
My cousin is non-verbal autistic and lives independently. People would call him very autistic or more autistic than most, but that's offensive to him. He has more qualifications than most people, he's extremely intelligent, but grew up in a time where high tech AAC wasn't a thing so missed out on a lot of opportunities and was treated as lacking capacity for many things despite his main barrier being he did not have a voice. He wasn't taught to read or write until much later, simply because he wasn't offered those opportunities from the start. He does require help with daily living activities, but he has his own flat, and family just care for him by popping in and following his schedule. He can leave the house alone and go to shops alone, and he has friends he meets up with and hobbies, and finds public transport easy to use.
There are those of us who can speak, but can't live independently. I can attend meetings, and have worked, to the point of having severe mental breakdowns. I require significant support with my daily living activities, yet this wasn't always the case, and as a child I'm sure some people would have called my autism mild, however I was just really good at masking neurological distress or was punished harshly for showing I was in distress and my distress minimised. I was diagnosed later in life with a dual diagnosis of autism and cPTSD and that is from a lifetime of unmet need.
So your daughter may well present mildly to you, but there's no guarantee that will always remain the case, and it might not be how she herself perceives it, particularly due to spikey developmental profiles.