I don't write much about mixed leagues, but I do answer as many questions as I possibly can.
Before analyzing the trade, I'd ask what the free agent pool looks like in this league. Carroll's roster-able in 15-team mixed, but depending on what other options are available, you might be able to do slightly better via your free agent pool. It doesn't impact the analysis much, but it does have an impact.
As far as the trade goes, I like the Fielder/Tulo/Tabata side better.
This seems like a classic "buy low" trade from the perspective of the owner getting Dunn, Uggla and Werth. The problem I have here is that even taking career norms into account, the owner getting Fielder, Tulo and Tabata wins slightly and there is more upside in Tulo and Tabata than there is anywhere else with this trade. Dunn, Uggla and Werth all should do better from here on in, but even if they do there isn't enough value here where you should feel compelled to take the gamble if you're the one getting those guys.
Categorically, the trade is mostly power (Dunn/Uggla/Werth/Espinosa) for speed/average (Fielder/Tulo/Tabata). If I'm getting the Dunn side and I'm desperate for power, I'd make the deal. If I'm getting the Fielder side, I'd probably make the deal.