The transliteration of モード for “Maud” is based on the name being pronounced in an accent that does not exhibit the cot-caught merger. In such an accent, the vowel in “Maud” is an open-mid back rounded vowel (or close to one), which sounds similar to Japanese お. However, the voice actors are almost all Canadian, and most Canadian English accents have the merger.
 
You can see the same sort of effect in, say, the transliterations of words starting with “auto-”, like “automation” (オートメーション).  
Regardless, モード is what most artists (and presumably fansubbers) have settled on.

@Dogman15  
モード is also Maud.

@Dogman15  
Whoops. I meant “Maud”, not “Maude”. Darn it.

Using a translator, I found that モード is “Mode”, and マウド is “Maude”. パイ is “pie”.

Maadu Pai

@Saski64  
Everything I’m looking at points to Maud being written as モード, and most people seem to write Maud Pie’s name that way too.

@Zharkaer  
why not?

@Saski64  
i guess i’ll just stick to my version

@Saski64  
To be honest, i don’t know, i don’t speak japanese, so i found this ( モード パイ ) version on internet.  
I could mix something up, my bad

@Saski64  
No

Shouldn’t Maud be written マウド and not モード ?

@Dogman15  
Yes, it says “Maud Pie”

Does the text say “Maud”?

…
 
Feels like she’s about to stab me. :/
 
… Or bash me with a rock. :P