Is any text which reports variant readings from other manuscripts a “critical edition”? And what is a diplomatic edition?

A text which reports all variant readings from various manuscripts without selecting a preferred one is a collation. In a collation, one typically reproduces also variant readings which are clearly wrong and will later be eliminated. Some of these details are irrelevant for the sake of the constitution of the critical text, but can be relevant for the history of the transmission. For instance, a manuscript often writing śa or ṣa instead of sa might be an evidence of the fact that the text was transmitted (at a certain point of its history) within an environment in which the dental sibilant was not distinguished, e.g., in Bengal. The same could be repeated, mutatis mutandis, about the use of retroflex ḷ for non Vedic words, the confusion between sounded and unsounded occlusives and so on.