Good, casual restaurant in Charlottenburg (nr Ku'damm)
Engelbecken is a good restaurant, but it is a bit far from Mommsenstrasse.
First of all, within a walking distance from Mommsenstrasse (at least most of it - it is a quite long street), you've got Savignyplatz and Kantstrasse. In Savignyplatz you'll find many nice restaurants, including the famous "12. Apostel" pizza/Italian place and Zwibelfisch, which is a casual, almost pub like, German restaurant. Kantstrasse has many of Berlin's most famous Asian places, including Good Friends (Cantonese) and Aroma (Dim Sum).
Marjellchen is right on Mommsenstrasse, and although I have never been there, I have heard it is a very good German restaurant with not too posh atmosphere.
We ended up at Leibnitz Klause, which was just a stone's throw from our Pension. I really liked it! The staff was very welcoming and "english-friendly". The restaurant was cozy and warm. I had the Konigsberger Klopse, which were meatballs in a wine/caper sauce. The meatballs were very tender, and the sauce flavorful (though I didn't need the extra bowl of sauce provided -- the dish was plenty salty). It came with a delicious plate of beet salad, and was around 14EUR. My father ordered the berlin-style knuckle, "eisbein." For what was the "mini" knuckle, this was an enormous portion of meat. Again, very flavorful (a tad on the salty side, but I found this to be true of all the meals we had in Germany, not just Berlin). Tender meat off the bone, and good sauerkraut. Overall, an excellent value at 12 EUR. I wouldn't call this a destination restaurant, but I'd recommend it to anyone who is looking for a lowkey meal in the neighborhood.