Reasons they shouldn't: 1. The Mavericks have more pressing needs
The Mavericks have yet to address their void of a starting caliber rim protector so the argument can be made that by expelling whatever assets that they need to part with for Hield, the Mavericks may be left thin in terms of trying to find an upgrade at rim protector.
The framework for a Buddy Hield deal would likely include Tim Hardaway Jr. as aforementioned, but without a true gauge on either Hardaway Jr. or Hield's value, it's difficult to determine if Dallas or Indiana would demand more compensation on either side of the deal.
If Indiana is asking for draft pick compensation alongside Hardaway Jr. for instance, it makes the deal questionable from Dallas' perspective given that they would be only getting a marginally better player in Hield compared to Hardaway Jr. Add that to the fact that the Mavericks would be exporting draft compensation to Indiana, and it makes a trade for Hield redundant if he doesn't vastly over-perform above Hardaway Jr. this season.
Obviously, the Mavericks sending a first-round pick to Indiana in a one-for-one swap of Hardaway Jr. and Hield would be the worst-case scenario, but even second-round picks are a hot commodity come the trade deadline in today's NBA.
The Mavericks will need to do their homework ahead of time if they wish to try and trade for Hield, as losing value on the deal would hurt their future flexibility and pursuit of a starting center. The trade would be especially catastrophic if Indiana demanded one of Dallas' younger assets like Jaden Hardy or Josh Green without adding anything else to the deal, so Dallas will have to be careful to not overpay for Hield if they make a deal.