When I was in university a lot of my colleagues dissed Mulligan and Zoot for their tone on soprano. Frankly, I always enjoyed the fact that , even as experienced players, they brought something new to the tonal palette of the soprano. My favourite soprano players are still Branford and Nick Brignola but Mulligan is in the list. I know from experience that playing a lot of baritone can help give you a full sound on soprano (as long as you don't have too resistant a set-up).
BTW, there's a great album from the 1960's called "Something Borrowed, Something Blue" with Mulligan, Art Farmer, Jim Hall, and Zoot. Mulligan plays alto on one cut and sounds freakin amazing. (I didn't like him on tenor, but he played great bari, soprano, alto, and clarinet).