Wasteful; (it's) a waste — expressing that discarding something or using it wastefully is regrettable.
食べられるのに捨てるのはもったいない。
It's a waste to throw away food that can still be eaten.
包装紙がきれいだから、もったいなくて捨てられない。
The wrapping paper is so pretty I can't bring myself to throw it away.
まだ使えるのにもったいないから誰かにあげよう。
It's a waste since it can still be used; let's give it to someone.
Too good; more than one deserves — feeling that something (a gift, praise, position) is excessive for oneself.
こんなに高価なプレゼントをもらって、もったいないです。
Receiving such an expensive gift feels like more than I deserve.
あなたが褒めてくれるなんてもったいない話だ。
Your praise is more than I deserve.
Impious or sacrilegious (archaic/rare use) — treating sacred things disrespectfully; now rarely used in this sense.
仏壇の前でふざけるなんて、もったいないことだ。
Fooling around in front of the household altar is disrespectful.