Evidence of freshwater lake that could have supported microbial life on Mars

Neutral pH, fresh water (not brackish), moderate temperatures. That's pretty exceptional anywhere in the Solar System that isn't Earth.

We've found RNA on asteroids? Not just amino acids?

Officially, no it wasn't built to detect life.

But there are cameras all over the thing.

So if something is scurrying around leaving tracks or skeletons, we should be able to see that just fine.

Or if it rolls up on a flower or a tree, that ought to be pretty obvious.

Not exactly. It was not specifically designed for detecting life, but it has a lot of instruments that could provide enormously strong evidence for life. For example, if you took a pile of mulch and put it through Curiosity's SAM instrument suite you could look at the data coming out of it and go "yup, that's a bunch of wood chips, unquestionably". But that would require quite a bit of luck. If there are only extremely small amounts of materials of biogenic origin Curiosity is unlikely to be able to definitively identify them.