Yes, that totally normal, but you need to understand some mechanics in SBD to make successful and efficient grinder:
1. Stacked Mobs.
Mob doesn't spawn in hordes, but instead stacked to reduce lag. At first the spawning would be slow, but once a mob spawned, the next spawning will stack the amount very quickly into that one Mob. Killing stacked mobs manually is like killing one at a time, one dead, next one spawns, so on until the number reached 0. But if you use external kill method like lava or height fall, the whole stack will dead. So, people tends to use cobwebs to stall the mobs from reaching killing point to give enough time for the mobs to stack as much possible.
2. Stacked Spawners.
Spawners can be stacked like mobs too, just right click the same spawners on the placed spawner and it will be stacked. This will reduce space needed and increase spawn rates.
3. Epic Spawners.
Killing normal mobs will give normal loots that can be sold at /shop. If you have a lot of same spawner, you can upgrade your normal spawners into a epic spawners (100 normal = 1 epic) that spawn epic mobs. Killing epic mobs will give stars and epic loots, epic loots cost much higher than normal loots and can be sold at /shop.
4. Mob hoppers.
If you kill the whole stack of mobs with external kill method (lava/fall), they will only drop their loots if there is a mob hopper. Mob hopper automatically collect all mob drops within the chunk it is placed (only need one). For more storage you can connect it with chests or place mob chests in the same chunk. Note that mob hoppers and mob chests not need to be connected or placed in certain places, as they pass the loots through "teleportation", as long as they are in the same chunk. Mob hoppers and chests can be bought from /shop, costs 100k each.
5. Exp, Heads, Mobcoins, Stars
Exp, Heads, Mobcoins, Stars will be dropped from manual killing, but won't drop by external killing method.
4. Redstone clocks
I don't think redstone clocks will be used in afk grinder, but if you do, I will tell you that all redstone clocks would not work in SBD. After a short time, all redstone mechanics that turns on and off rapidly would be shut down. All other redstone mechanics will still work fine. That means most automatic cobble gen won't work.
Note: If you build a normal spawner room without spawners (Mobs do spawn on dark places), the spawn rate is too low and not recommended.
Some other normal spawning requirements are still applied, but some might have been changed. There might be some changes that I don't know.
Hope this helps and good luck. Just ask if you have any question.
EDIT: I just re-read the question and realize you are not saying "mob grinder" specifically. If you mean afk cobblestone gen, I think it's very difficult. For afk generating, redstone clock is not working mostly (some people use slower clock). For afk breaking like abusing F11 glitch, holding attack button with something, or using autoclick is strictly prohibited and may result in ban.