Make the question titles useful for real people first. Including "for Ubuntu" in every post would get really tiring and lead to less user participation if enforced — much more disastrous than a small loss in SEO, even if that exists.
Compare to Stack Overflow, which has questions about many different programming languages, yet it's still much more common to not include the language in the title — and the titles are usually more clear and more useful because of it. Remember that tags are displayed alongside the question title in many cases.
The analogue for us is not including the specific Ubuntu version in each question title: keep the titles shorter and more managable. Even if this only reduces clutter and text wrapping on the homepage, that would be worthwhile.
For what it's worth, "Ubuntu" is already included in every page title ("Question title - Ubuntu - Stack Exchange") plus numerous times on the page. Any search engine that can't associate Ask Ubuntu posts with Ubuntu won't have many users.