A look through my sources suggest that there isn't really much said on what others thought of Tamar. This may be because she was mostly concerned with domestic affairs (while she did expand Georgia, I would argue it was mostly related to the former), and the relative unimportance of Georgia at the time.

That said, I found one brief mention that could give an idea. Donald Rayfield's Edge of Empires: A History of Georgia (2012) has a whole chapter devoted to Tamar, and notes that after Saladin conquered Jerusalem in 1187 she sent two missions to negotiate both the safe passage of Georgian pilgrims to the city, and the safety and continued tax exemption of the Holy Cross monastery, which was run by Georgians. That Saladin undertook these negotiations, and accepted them, gives some idea that he considered Tamar someone worth dealing with, though it's hard to read into what amounts to a small paragraph in the book.

The difficulty here is that there is a dearth of literature on Georgian history in English, especially medieval Georgia. No one really writes about Tamar, or Davit Aghmashenebeli, and my Georgian is not strong enough to read any of their works, which is what is probably needed for a more comprehensive answer like this (I had a far more detailed reply written up, but my computer crashed on me right as I finished, so this somewhat truncated reply will have to suffice).