Can we replace an attitude indicator with a ball half full of liquid?
Nope.  In a perfectly flown turn, the ball of liquid would appear to indicate level flight.

You might argue that it could replace the "ball" of a turn &amp; bank indicator, but that's about it.

It would also be tough to read in turbulence.

See also this YouTube video showing Bob Hoover pouring iced tea while flying the plane through a roll, even while inverted.  Your liquid would behave similarly.