Not IMO. In fact I thought the heat overachieved in getting to the finals. I'm a big Lebron/Wade fan and Bosh is a good player, but I've been skeptical about them since the start. They are, in essense, a donut team... lots on the outside but a big hole in the middle. Championship caliber teams in my opinon always have a truly good 2-way big man (or two) anchoring the defense and offense too. Gasol/Bynum, Garnett/Perkins, Ben/Sheed, Shaq, Duncan, Robinson, Olajuwon, Kareem, Parish/McHale, Moses, etc. Basically the only team that ever truly won without at least one dominant big man were the 1990s Chicago bulls (and even then, guys like Horace Grant and Dennis Rodman were big time impact players). I don't consider Bosh to be a dominant big man... he's more of a 6'10 small forward.
If you look at the Mavs, they had Tyson Chandler rebounding and intimidating everywhere, and even Dirk is an impactful defensive big man although not a shotblocker. The heat were outmatched up front, and it showed in things like defensive rebounding and easy layups and outside jumpers off penetration from the mavs. If Boston didn't trade Perkins (or if Shaq was healthy) I would not have expected the heat to beat them.
I do think the heat COULD have beat the mavs, but only based on making tough jump shots and doing it for four games. If you switched Tyson Chandler from the Mavs to the Heat, then I'd definitely say it was a big upset. Right now they're still an incomplete team though.
