I noticed that a lot of the German posters for showa kaiju films you post tend to have Frankenstein in the title. Is there a reason they did this? And did it affect the movie's plot at all?

In Germany, a lot of the films were re-titled with the Frankenstein name in them to make them appeal more to German audiences, with whom the Frankenstein myth is very resonant. I’ve heard from different sources that this was also possibly carried over into the dubbing itself, and sometimes monsters (in particular, Godzilla’s enemies) would be explained as being creations of Doctor Frankenstein. I’m not entirely sure either way, though. spacehunter-m is probably the best person to ask about this!