Yes you could I guess but I am not sure I would do it, for two reasons:
I made a quick ideal case scenario:
which leaves you with ca. 2h by my calculations to see the city. However that was a best-case scenario and your calculation could go wrong at any time above:
- delays of your incoming flight
- queues at immigration and/or security
- questions at immigration
- once the tracks where blocked on my train into the city and I waited 30min on the train stopped at a station before resorting to alternative means
- get lost in Frankfurt
I am a fairly risk-averse person and honestly would not do it unless I had an important errand to run and could fall back on taking a taxi back in case, but this is up for your judgement. If you do go, plan ahead every detail, i.e. what terminal you arrive, how to get into town, where to get off, where to walk around, when and how to get back, ...
Personally I don't find Frankfurt that attractive for a visit and if I had a friend come to Germany and could show them only one place, it would be among the last to come to my mind, but hey, you might have only this chance to see Germany and it is better than nothing. Have a look at what to see beforehand and plan how to get around.
Alternatively, sometimes (and only sometimes) you get pretty good views of the "skyline" at take-off and landing, see this question of mine.