もんだな

that's quite …

→ もの
phrase Rare #22,498
that's quite … A casual, often masculine-sounding expression of surprise or reflection about something. もん is a contraction of もの.
JA: ずいぶん変わったもんだな。
EN: That's changed quite a bit.

優しくなったもんだな、キダ

You've really softened up, Kida.

— アトランティス 失われた帝国 (2001)

「そしてテーブルに座りたいもんだな。」

And I’d like to sit at a table.

彼を信じるとはばかなことをしたもんだな、君も。

You were a fool to trust him, weren't you?

俺たち以外にも忙しいやつが居るもんだな

Looks like we're not the only ones who are busy.

— GHOST IN THE SHELL (1995)

ひでえもんだな

That's awful.

— High and Low (1963)

うまい言い訳もあるもんだな

That's quite an excuse.

— ストーカー (1979)

やけに弱気になったもんだな

You've gotten awfully timid.

— イノセンス (2004)