Ernst Mach died in 1916.
I don't remember any historian of science mentioning Ernst Mach commenting on GR, making it unlikely that Mach ever commented on Einstein's GR ideas
On the history of Mach's principle:
Historians of science describe that what is now known as 'Mach's principle' was first introduced by Einstein.
My main source is John Norton. In particular the article: Mach's principle before Einstein
Historians of science emphasize that Ernst Mach adhered to an extremely austere philosophy of physics. Just how austere can be seen from Mach's position on the question whether matter consists of atoms. Mach was among the last of physicists to acknowledge that the existance of atoms had been proved beyond doubt. That is: Ernst Mach argued for an extremely high burden of proof. Mach argued: if you cannot prove it it is speculation and not part of science.
In his book on mechanics Mach criticized Newton's notion of absolute space. Speaking more generally Mach argued: since we have no means to investigate whether the inertia we observe locally is due to distant matter (the fixed stars), this is a speculative thought, and hence not part of science.
As we know, Einstein did the very thing that Ernst Mach condemned: Einstein made it into a principle: Einstein's Mach's principle.
While Einstein's Mach's principle was a strong source of inspiration during the development of GR, it was later abandoned. Historians of science describe that within five years or so after completing GR Einstein abandoned Einstein's Mach's principle. This is manifested by the fact that Einstein ceased mentioning Einstein's Mach's principle. In the first years after completing GR, when writing an article about GR, Einstein still asserted that in GR Einstein's Mach's principle was fully realized. Over time Einstein stopped making that assertion.
Explicit confirmation of Einstein abandoning Einstein's Mach's principle is found in correspondence between Einstein and F. Pirani (letter, 2 february 1954) "Von dem Mach'schen Prinzip [...] sollte mann nach meiner meinung überhaupt nicht mehr sprechen." ("In my opinion we should no longer talk about Mach's principle at all." )