Does マシ come from English? Or is it a semantic emphasis?
まし is not a loanword. It is actually 増し, the noun form of verb 増す (“to increase”). However, it is not usually written in kanji, probably because the meaning diverged widely from the original verb 増す.
If you follow the standard orthography, there is no reason why まし should be written in katakana. However, it is true that many people write マシ in katakana. I do not know why.
The following is my speculation about a possible reason. Sometimes words are written in katakana to avoid a large chunk of hiragana and make them stand out in a sentence. For example, in the following two sentences, the latter may be easier to read.
This may (or may not) be a reason why many people write マシ rather than まし.