"Frankly" is fine, but one has to be careful with "remarked". "Remark" as a verb is stylistically difficult to use in a construction like "She frankly remarked that …". It could sound unnatural. It is often safer to use "said" or a more "loaded" verb such as "admitted". The "frankness" does not have to be "evidence-based". It describes the listener's perception. My advice is to avoid using "remark" as a verb until and unless you are absolutely certain that it is the best verb to use. It is tricky. Let's look at the full sentence or paragraph. We could advise you further then.

yes, "she frankly remarked" is fine or "she remarked frankly"