My impression is that the TRS-80 line was... adequate. They were serviceable systems, but there was nothing terribly noteworthy about them. As mr_me pointed out, they actually led in sales for a few years. But between IBM entering the market on the high end and Commodore dominating on the low end with the Vic-20 and 64, they quickly lost market and mindshare at the turn of the decade.
The CoCo series was also adequate, but couldn't compete with Commodore and Atari as gaming computers.
For my money, the Tandy 1000 line was their biggest success. They took the promise of the PC Jr, and made it work.
If sales had not been limited to Radio Shack stores, the Tandy computers could have been a much bigger deal. But we'll never know.
The Digital Antiquarian has covered the TRS-80 pretty extensively, specifically around how it helped usher in the nascent interactive fiction genre: https://www.filfre.net/tag/trs-80/