There are a LOT of things you can say about Freddie, not all of them great, but that "he's not a great live singer" is absolutely not one of them. I literally have never heard that criticism EVER. Do they always sound like the record, no, but listen to those "Live From the Rainbow" shows. I also think that it was more than "frontman" that has Queen widely regarded as the highlight of Live Aid back in '85.
Finally, I always point to the Freddie Tribute show, where "Queen songs" - specifically the vocal parts - made some of the most well-regarded singers in rock seem like bar-band wailers.