Change of interest for PhD after BA & MA? Does it look 'academically' immature?
There's nothing immature about changing research themes from MA to PhD. It's very common especially in philosophy (philosophy PhD myself).

That being said, there's something ill-advised in writing I now want to not do anything with German Philosophy at all anymore.

Instead, It's best to explain your new found interest and talk positively about it and the sort of interesting research you want to do on that. Worded another way, having good reasons for wanting to focus on political philosophy will make you sound mature.

Similarly, what's wrong with telling someone that you are interested in pursuing research lines similar to the ones they do ... when you are interested in pursuing research lines similar to the ones they do. Instead, it's precisely what you should do and be able to do at the PhD-level (note you don't need to make it seem like fawning over the person if you focus on the idea).

The one thing that's slightly troubling is what I mentioned above. Unless political philosophy is being construed very narrowly, there's quite a bit of German philosophy that interacts with it... (the point of this paragraph isn't to argue with your new-found interest -- it's to highlight that mentioning your old interest negatively inspires just this sort of reaction and focuses things on what shouldn't focus on).