The jacket is certainly a quirky clothing choice, but throwback intentions aside, I don't think it undermines Blofeld's character. Must he be taken less seriously because he dresses in a style most people outside of India don't? It's not like he's dressed in jeans and a t-shirt; he's dressed formally, only taking a page from another culture. I think your claim that the attire, coupled with the scar and the cat, reflects a lack of creativity (or perhaps a lack of initiative) is more significant.
I think he would be taken less seriously, just as Craig's Bond would be more risible if he wore bell-bottoms like the ones Moore sported in The Spy Who Loved Me. The Nehru jacket isn't there to show Blofeld has an international dress sense (wearing loafers without socks suggests this Blofeld aspires to dress like a Western jerk)--it's part of a set of props designed to link this Blofeld with that of YOLT, despite their complete difference in personality.