Truth is, it appears to be both. Per usual, too much appears a negative, too little, also a negative
Supplement companies are constantly pushing mTOR as the magic pathway to building muscle, and various net guru type talk about keeping mTOR activated, with heavy marketing of Leucine, BCAA, etc. to keep mTOR activated. It’s unlikely that keeping an anabolic pathway such as mTOR active is a path to longevity. It’s interesting to note, cancer researchers, anti aging researchers, etc are looking for ways to limit/control mTOR.
Here’s an article on the topic by a researcher in the field worth a read. Obviously, it’s far from conclusive at this time that healthy people intentionally activating mTOR to assist in altering bodycomp etc, but it’s also not difficult to fathom that pathways also responsible for cellular growth, cell division, etc being activated constantly could have long term repercussions. We need mTor and too little also also clearly a negative, and like growth factors such as IGF-1 the risks/benefits will appear as a U shaped graph, where too much or two little increases the risks for various pathology best avoided (1). Interestingly, drugs that lower mTOR have been found to increase life span of some animals for example.