Is this a nice or a mean thing to say about someone? What does ‘mediocre’ mean here?

It's impossible to know what that specific person meant, because they might not know what 'mediocre' really means. But I would take it as an insult if someone wished me a 'mediocre' life. It means dull, uninteresting, average, with no excitement or colour.

If their life was previously poor, it would be an improvement.

“Mediocre” means neither good not bad.

Many would take this as something mean based on the sense that mediocre means nothing special.

It is a mean thing, and mediocre means in the middle below average.

On a scale of F to A+, think D+ to C. Synonyms are plain, ordinary, basic, generic, dull. With 1 as terrible and 10 as great, 2.5 to 4.5 is mediocre.

A movie or television show that you call mediocre means that you do not believe it is worth money to see it and not something to look for, but it is not terrible.

Mediocre is not good nor bad, but in considering options, we prefer better.

Excellent, Very Good, Good, OK, Mediocre, Bad, Terrible/Awful

It's a mean thing to say. Mediocre, not very good, not very bad.

Average, ordinary, dull