Or, preface your statement with many disclaimers, such as: I don’t think she’s less Frum than me, but I wanted to go a certain route that I see her dressing doesn’t match.
I would advise however that you speak this over will relatives who’s opinion you trust. Perhaps what you notice is a minor issue that she can change on demand. She might be dressing up to a standard that she feels would be Shidduch inducing. Once she is engaged to someone who does not appreciate that way of dressing she might just switch. This needs someone who knows both of you well enough (besides the Shadchan).