What is Unicode?
Computer operates by assigning a code number for each English character. For example, letter A has the number 65. Now we have separate code numbers for all Malayalam characters - in fact, for all languages of the world. This numbering scheme is called Unicode. In Unicode, Malayalam first letter 'a' (അ) is given the number 3333. Simillarly different different numbers for rest of Malayalam alphabets. The fonts that understand these new code numbers are called Unicode fonts.
Fonts like ML-TTKarthika, Thoolika etc are not Unicode fonts. They used the code numbers assigned for English alphabets for Malayalam. That is not any standards based. So a document written in ML-TTkarthika can not be processed by other applications like google search.