If there was some kind of forcible conversion in this specific context , (and I have no clue if there was or wasn’t, since I don’t know really much about Mother Theresa ) the Hindus that I knew (bear in mind this is a very small number ) who grew up in Kolkata around that time would have said “Who am I to object to a ‘forcible conversion’ (in that context), at least she was taking care of them!” (I.e., if she’s doing something charitable and nobody else is stepping up to the plate, then they have no business criticizing her.). In fact, I’ve heard someone say exactly this.
They would be less excited about this happening in some other contexts though.
As an aside, most Hindus don’t object to conversion as long as it’s done voluntarily in good faith. The objection many of those have to conversion attempts (some, not all) in India, is that it either seems like it’s not in good faith, or often involves a misrepresentation of Hinduism to proceed.
Hindus are pretty used to their belief system being misrepresented all over the place, but is particularly frustrating in the country of its origin.
Also the laws are different there, so the church state separation that people take for granted in the west doesn’t exist there for Hinduism specifically, though it exists for other faiths in India.
So, for instance, you’ll have people of other faiths (or who are atheistic)making decisions about what happens in terms of temple policy, which in the west would be seen as a church issue. Whereas this wouldn’t happen for any other faith tradition in India. Hence people are upset for political reasons.