mäßig
mediocre
Of mediocre to below-average quality; not especially good.
Der Film erhielt nur mäßige Kritiken.
The film received only mediocre reviews.

mediocre / moderate
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mediocre
Of mediocre to below-average quality; not especially good.
Der Film erhielt nur mäßige Kritiken.
The film received only mediocre reviews.

The word 'mäßig' (mediocre/moderate) is closely related to 'das Maß' (measure). It describes something that either does not reach a certain standard (mediocre) or stays within a reasonable measure (moderate).
Der Arzt empfahl ihm mäßigen Sport.
The doctor recommended moderate exercise to him.
'Mäßig' is very often used as an adverb to modify a verb or another adjective. In that case, it means 'not very' or 'only to a small degree'.
Das Konzert war nur mäßig besucht.
The concert was only moderately attended.
If 'mäßig' (mediocre/moderate) refers to performance, quality, or a feeling (e.g., film, success, interest), it means 'mediocre'. If it refers to an amount or intensity (e.g., consumption, exercise, temperature), it means 'moderate'.
Seine schulischen Leistungen waren leider nur mäßig.
His school performance was unfortunately only mediocre.
In the meaning of 'moderate', 'mäßig' is often used in advice about a healthy lifestyle. Typical combinations are 'mäßiger Alkoholkonsum' (moderate alcohol consumption), 'mäßige Bewegung' (moderate exercise), or 'mäßiger Genuss' (moderate enjoyment/consumption).
Ein mäßiger Weinkonsum kann Teil einer gesunden Ernährung sein.
Moderate wine consumption can be part of a healthy diet.
good
'gut' and 'mäßig' can both evaluate quality. 'gut' means that something is positive or was done well. In this context, 'mäßig' means more like: not especially good, perhaps only average or somewhat disappointing. So the contrast is not always very strong. 'mäßig' often sounds milder than 'schlecht'. If a result is truly positive, 'gut' fits. If it is just so-so, 'mäßig' sounds more natural.

Das Essen im neuen Restaurant war nur mäßig.
The food at the new restaurant was only mediocre.
Julias Vortrag in der Schule ist gut und leicht zu verstehen.
Julia's presentation at school is good and easy to understand.