If you can overwhelm and underwhelm, why can't you just plain whelm?

This is such a strange question. I think you're trying to be cute, but it isn't cute when the cuteness is mistaken. Of course you can whelm. You can even "just plain whelm." Why couldn't you?

Here's the definition:

" d. To throw (something) over violently or in a heap upon something else, esp. so as to cover or to crush or smother it.

4. trans. a. To cover completely with water or other fluid so as to ruin or destroy; to submerge, drown; occas. to sink (a boat).

b. To bury under a load of earth, snow, or the like.

5. transf. To engulf or bear down like a flood, storm, avalanche, etc.; hence, to involve in destruction or ruin. a. in material sense.

b. in immaterial sense.

Hence whelmed ppl. a., {sm}whelming vbl. n. and ppl. a."

.

.

You probably can, but whelming is just so ordinary that there's no need to use it. When you go to extremes on either side, then it is worthy of notice, but as far as the middle ground goes - forget it.