In the park is Mary. (passive sentence) read below.
it will be used for stating that Mary is in the park. It is actually a passive sentence because the
subject "Mary" comes after object "park" or you can say at the end of the sentence. Whereas in active sentence, the subject "Mary" will come before the object "park".
The active sentence of this would be this:
Mary is in the park. (active sentence) (look subject is now before the object).
Moreover, >*In the park Mary is*
It is actually not a sentence but, a phrase or you can say "a part of a sentence" because it not stating something completely. For ex. a sentence would be In the park Mary is playing.
Therefore, it wouldn't grammatically wrong sentence but, a incomplete sentence.
If you dontdon't know about active and passive voice, proceed to this site : EnglishPage or ask in the comments and if you didn't understand something, you can ask me that too with no hesitation. I'll surely reply you or edit my answer.