I… really dislike that song too. Though I still feel it's more frenetic than the new one in ways. As a pet peeve, it kind of irks me when people say Sonic 3 & Knuckles and what they really mean is just Sonic 3. Because the Act 1 boss theme from Sonic & Knuckles is amazing:
!
For two games that were basically supposed to be a single long game, I seriously don't understand how Sonic 3 got so many thing wrong (IMO) where as Knuckles did it all beautifully.
Sorry, I forgot about that theme. If I remember they were different, I would have specified Sonic 3 only.
But I agree with you though. The tone of both the Sonic 3 and Mania mini boss themes feels dissonant with what I would associate with Sonic.
I personally still like some of Sonic 3's OST. Mainly just the File Select, Angel Island, Hydro City, and Big Arm themes. And maybe Launch Base, but we're never hearing that again because of the MJ composer controversy. I respect Ice Cap, but… eh.