I started single track downhill mtn biking on a rigid frame way back before suspension and have to say that on rocky, root infested trails with drops, it would beat the crap out of me in a couple of hours. My body was the shock absorber with bent elbows and bent knees out of the saddle and still it was brutal, and I was in excellent physical condition. Was I over biked for the terrain I chose to ride? Definitely not. If I just used that bike to noodle around on rail/trail conversions it would have been perfect.
To lesson impacts, I grabbed the first front suspension hard tail, and that made a significant difference in enjoyment and duration. Asking the same question, over biked? Nope, because staying out of the saddle to use knees for shock absorption still became tiring and my back still took a beating.
fifteen years ago I bought a full suspension bike and OMG what a difference in comfort, ride ability and longevity of rides. Taking the same trails as my previous bikes was far more enjoyable. Now I must be certainly over biked! Not so fast. The new bike was so much more capable I was now taking the trails faster and doing much more technical descents. In other words, It raised my game to take on more challenge and push the limits. Mountain biking is about conquering technical challenges, conquering fear, and bike handling. With the wrong tool, just like taking slalom skis to a downhill course, it can really is not as enjoyable.
OTOH, riding a full suspension mtn bike on a MUP or rail/trail is definitely “over biked”.
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Road 🚴🏾♂️ & Mountain 🚵🏾♂️