First, like @Tim said, 6/8 is a whole other conversation, it’s 2 pulses per bar and 3/4 is 3 pulses per bar. They each have a distinct and different feel from each other despite having the same number of notes per bar so I’ll just focus on the 3/4.
Good music is good music and is also subjective so a ballad can be in whatever time signature the composer decides he/she wants to write it in, 3/4, 4/4, 6/8, 9/8, 12/8, etc. We decide as listeners whether they’re good songs to us based on the content more than the time signature.
Historically music in 3/4 time has been composed at many different tempos from ballads to blisteringly fast in many different genres such as classical music, opera, jazz and musical theater to name a few. The reason I believe that rock and pop music uses 3/4 mostly only in slow songs is because upbeat rock and pop music is so ingrained with the backbeat on 2 and 4 in 4/4 or 12/8 time that a fast waltz in a rock or pop style would be unusual. You can put a backbeat on a faster 3/4 groove either on beat 2 or 3 and it will work (and has worked) but most people are just used to hearing that big 2 and 4.