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Are you

a cloud,

because

you accumulate a lot of water and then drop it back to Earth.

No hooray because

its raining.

Are you

a towel?

Because

towels can be plain

they gain weight when used, but not in a bad way

the next day they weigh less because the water evaporated

On similar note to @A.Chung's answer, what about

A glass savings jar

Look at me, plain as can be.

The jar is clear, and the money inside can be seen plainly

Now leave me be, don't disturb me!

Once you put money in you tend to leave it there for a while

Over time, it's hard to say. 

I gain weight 

Not in a bad way.

The jar gets heavier the more money you put in there which is a good sign

Now go see me, inspect me every which way.

You will find, in no small way

All that extra weight is gone today.  

No Hooray 

When you open the savings jar to get the money out and count it then all the extra weight is lost. If you need dip into your savings then there might not be a reason to cheer.

Are you

Dough

Because

It's plain, you leave it alone for a while, so it rises and gains weight. When it's done rising you poke (inspect) it all over and the extra weight is gone.

This feels like a stretch but

Are you

Eyelids?

Because

You grow heavy when not looked at (awake). Then once i am asleep, during REM, you are looked at every which way. And when i wake up, all that weight is gone.

You could be

a tree.

Because

Trees gain weight over time as they grow leaves and fruit, then lose it all quickly when they drop their leaves in the fall (all over my yard ...........)

Are you

eyes?

because

You get sleepy during the day, weight is added, then when you sleep and weight is lifted

Are you:

Static electricity?

Because:

Static electricity can build up on items over time, without changing the look of an item, so it looks plain. It's winter in the US, which means the atmosphere is dry and more prone to static electricity building up. Electrons have weight, just not much. Static electricity isn't usually a bad thing, as long as it can be discharged in a controlled manner. Inspecting something usually means touching it, so you've discharged the static electricity by touching, losing all the weight of the electrons.

No Hooray:

You just gave yourself a shock that probably didn't feel very pleasant. You may have also discharged the static into a piece of electronics, and now it doesn't work.

Are you:

Wallet

Because:

gain weight=money gain,extra weight gone = money used/loss,after inspect you may use money to buy something

No Hooray:

Never happy when money gone

A not totally serious answer, but are you a

Particle?

Because:

Particles have the ability to behave as a wave. This way they can pass through multiple openings in a barrier and meet on the other side (gaining weight). However, this meeting can only occur when not observed. When observed these electrons can not behave as a wave, because you observe them specifically as a particle. Thus inspecting them every which way will have you find it without the meeting taking place.

No Hooray:

The mere act of watching done gone and botched up your experiment!

Bit of source material:

https://en.wikipedia.org/wiki/Observer_effect_(physics)

Are you

Weighing machine

Because :

It can be plain. You look at it everyday. It looses and gains weight rapidly.

It could be the day time. You just leave it be, the day goes longer and longer, then it goes the other way and all it has gained is lost.

I'd have to guess:

A caterpillar

Because

They get bigger and heavier until they make their cocoon, and when they shed their cocoon, the weight stays with it...

Are you

The moon

Because :

The moon can be plain. You look at the moon the now what day it is. It grows and shrink with time.