Is mixing troop types a mistake?

Yes — but careful: you can't mix types within one army. An army fields a single type, carried by its 3 dragons. The "dilution" mistake is really about switching type constantly from one army to the next, or from one fight to the next.

Why that's bad:

• Research on the type you field is your biggest multiplier. Spread your armies over 4 different types and you have decent research on none of them.
• Same for production: making a little of everything means never having enough to fill to the max.

The right reflex: focus on ONE main axis (your most researched type plus the greenest one on your roster) and run most of your armies on it. Only branch out with a real reason — scouting a small tile, for instance.