Solution:

Eye

Solution:

I am a mobile phone.

Solution:

A Mirror

Solution:

Fishing Net

Solution:

A Party

Solution:

A Clock

Solution:

A thermometer

Solution:

Dice (6 sided)

Explanation:
> I am always 1 to 6: numbers on dice are from 1 to 6.

> I am always 15 to 20: Sum of all the visible sides (1 top and 4 on the sides) always add up to 15 to 20.

> I am always 5: Five sides are visible.

> I am 21 when I am flying: When we throw the dice, we can see all 6 sides for a small moment which all add up to 21.

Solution:

Eyes

Explanation:
We have black but we are not tea seeds - Pupil of eye
We have white buts we are not lime - sclera of eye
We have water but we are not a river - general tears in eyes to protect eyes.
We have feather but we are not birds - lashes.

Solution:

Either Rain Cloud Or Air

Explanation
=== Rain Cloud ===
I don't have a mouth but can roar - Thunder
I don't have eyes but can cry - Rain
I am really good - Rain is good
but bad sometimes - Storms
Without me, all will die - rain is the necessity for agriculture and human being

=== Air ===
I don't have a mouth but can roar - Wind
I don't have eyes but can cry - Moisture
I am really good - Rain is good
but bad sometimes - Hurricanes
Without me, all will die - Oxygen