I would definitely take a Maggie
seriously. And also a Tess
and possibly Ellie
. It depends a lot on the context though. If I meet a doctor or see a scientist or historian on TV (don't meet them in real life) then I take into account what they have to say and how they present themselves, as opposed to their name.
I think the reason people struggle to take these names seriously is because they have only recently come into fashion. I don't know any grown up Millies, Tillys, Evies, Maisies, Emmys, Lexies etc. So I think people get a cutesy, fluffy vibe from these names because they only see them on small children or teenagers. In 30 years time the world will be full of doctors and scientists called Evie
and we might have a different image from them.