using dryer 240v outlet for 125v kilm
No. Just no.

Current

Your kiln specs are for 20A. Your dryer breaker is (or should be) 30A. 30A is a lot more than 20A. If you had a failure where the kiln pulled 30A for a while, the breaker would not trip and your kiln might be damaged...or start a fire.

Ground/Neutral

Your kiln needs hot, neutral and ground. A 6-30 is a 3-wire connection. It will likely have hot, hot, ground, in which case you don't have a neutral and would then be piggybacking neutral for your 5-20 on ground, which is not to code and not safe.
Your dryer receptacle is actually more likely a 10-30, which is an obsolete-but-grandfathered type with hot, hot, neutral, in which case you don't have a ground and would then be piggybacking neutral for your 5-20 on ground, which is not to code and not safe. It is considered &quot;safe enough&quot; for dryers but your kiln does not fall under that limited exemption.
Your best bet to avoid running a new cable from the panel to the garage would be to see if you can retrofit ground. There are rules for doing that, but if you can legally do it then you could:

Replace the 10-30 with a 5-20 (no dryer any more) and replace the 30A breaker with a 20A breaker.
Install a subpanel. Not sure if you can use retrofit ground for a subpanel though - Harper or one of the other pros can answer that (and fill in 100 other details I missed). Then connect a 14-30 (replacing the 10-30) for the dryer, a 5-20 (for the kiln) and other circuits as well. As long as you don't run the dryer and kiln at the same time, this would probably work quite well.

Double-check the specs - and safety - of the kiln
The specs are a bit confusing:

120V
20A
2,300W
5,940 BTU
10 AWG
NEMA 5-20

20A specifications: 20A (obviously), 5-20, 5,940 BTU (1 kWh = 3,412 BTU, so 5,940 BTU = 1.74 kWh = 14.5A @ 120V, which fits just fine on a 20A circuit)
30A specifications: 10 AWG (it is perfectly fine to use 10 for 20A circuits, but code normally only requires 12), 2,300W (2,300W/120V = 19A and since this is continuous usage it needs a circuit rated for higher current - and the next typical size is 30A)
Just doesn't add up... That could be laziness, specs not up-to-date, etc. Or it could be a sign of poor and possibly unsafe design.