I'd say so.

Android has gotten more people running open source software, but it hasn't done much to improve the ecosystem. There's not much coming back upstream other than some kernel updates.

Sure. Android is basically Linux with a proprietary GUI and a riped-off of Java runtime.

Plus, it's mostly used for proprietary mobile device offerings by big corporations.

Not many open source projects benefit by Android's existance.

Depends how you mean it, really. Android has had an impact, and happens to be open-source (albeit not operated like a normal open-source project; it's closer to a code-dump, as with Apple WebKit in the early days). However, if Android was not open-source, but had source shared with OEMs, would its impact really have been substantially different? I suspect not.

However, if Mozilla, clang or WebKit had not been open-source, their impact would have been very different (and much less).