For my perspective, I did my undergrad and a master's at BYU. I then did a PhD and postdoc at top research universities including an Ivy league school and now I'm a professor at another, mid-tier university. With regards to research, I would say that BYU's faculty are probably about average to good. It will vary a lot from department to department and it is hampered a lot by the focus on undergraduate education. For that reason, it is hard for BYU to attract superstar faculty in research. Plus they don't really do incentive pay like other universities do and so the really good research professors won't get paid higher at BYU. So from a research perspective, BYU is pretty far from the top and is also not the top in the state. In fact, I think I would put it below USU in terms of total research.

However, in terms of undergraduate research opportunities, BYU is one of the best if not the best. While there might be cooler research happening at R1 universities, the big shot professors doing cool work have their pick of PhD students and postdocs. Undergrads are often mostly useless for research and can take a lot of mentoring to get anything out of them. So it can be quite hard to get a research position with a top professor unless they have some NSF quota they need to fill. BYU does remarkably good work with undergraduate research. This is a big reason for why a lot of BYU students can go on to top PhD programs.

Finally, being good at research does not directly translate into being good at teaching. So even if the professors are "average" with respect to research, I don't know that you can say that they're average with respect to teaching, which is a harder thing to measure. BYU professors will do more teaching than professors at top research universities. So they get more practice. Plus they do more research than places like BYU-Idaho, UVU, SUU, etc. I do believe that research can help improve your teaching to some degree. So I think you can make some arguments that BYU professors are better than average at teaching.

But I think there is one more big thing when it comes to teaching. A lot of other posts in this thread have mentioned that the quality of the student body is very high at BYU. This has a major effect on how you can teach a class. To a large extent, you have to teach at a level your average (or perhaps even below average) student can understand. If the average is much higher at a place like BYU than, for example, the U, then you can teach the class at a higher level. So even if the professors are "average", the overall education is better because the professors can teach at a more advanced level. The work ethic of the student body also plays a role. At Ivy league universities, you will have a similar or even higher quality of students. But there's also a lot of entitlement (based on my interactions with professors at these schools). A lot of the students worked extremely hard in high school and are now away from home for the first time. They're ready to party and relax some. Plus they've always gotten A's so now they still expect the high grades. You might get that some at BYU but probably to a much lesser extent.

TL;DR: by a fair amount of measures, you can argue that BYU faculty are "average". But the undergraduate research opportunities and high quality of the student body push it squarely into the above average range for undergraduate education. I would even say that it is fair to make comparisons to Ivy league schools. This changes when we talk about graduate school though. BYU is very mid-tier in that sense and nowhere near the top with a few exceptions (e.g. some of the business programs).