Learn English – Something between “satisfactory” and “good”

Well, there's isn't any dictionary word for that. The "bearer of bad news" is the common idiom.

Some sources say that you can call such people "Cassandra", which I go against. You see, she didn't bring bad news, she simply made true prophetic statements that would not be believed.

Other sources indicate that "harbinger" is likely possible. "Harbinger" is someone who shows that something is going to happen soon, especially something bad.

There's also "The messenger" (taken from "Shooting the messenger") - is a metaphorical phrase used to describe the act of blaming the bearer of bad news (the messenger).

I might use easygoing, which is synonymous with undemanding. M-W lists it as any antonym of fussy, which can be applied to tastes in food or clothes.

One definition that fits this context is:

easygoing (adj) relaxed and informal in attitude or standards

An example sentence might be:

Diane is really picky about what she eats, but her sister Jill is much more easygoing.

An idiom that might work is go with the flow, which Macmillan defines as:

go with the flow (phrase) do what seems like the easiest thing in a particular situation

This may not a precise fit, but it could work depending on what you were trying to convey:

Every time we go to the mall, Diane is really picky about clothes, but Jill just goes with the flow. She'll pretty much agree to anything we suggest.

The phrase go with the flow suggests a calm and accepting attitude.