As a german, I wouldn't have known that those are supposed to be german Umlauts if you didn't put it in the text. Having two separate dots on top of the letters instead of a single line would be much better.
I'd go with wide spacing like the Minecraft font, some slanted lines like on top of the n would also work. But I'm not sure if those would confuse non-germans.
Edit: You also don't seem to have a capital version of ß, which does exist and is a little different in most fonts. Due to the lack of space in a pixelated font I'd just copy the non-capitalized version to make sure nothing ends up falling back to a different font.