Downthrow is very techable, yes. The only characters that have to tech it are fast fallers and mid-floatiness characters (i.e. shiek, mario, etc.) at low percent, others will just jump out before hitting the ground. The throw is also slow enough that it's relatively easy for competent players to tech it. If they tech in place, you can still get a usmash if you wait slightly (for their tech invulnerability to fade). If they techroll, however, you probably will be unable to directly punish, as pikachu likely won't recover quick enough from the throw animation to dash->usmash before the opponent can shield.
Downthrow isn't guaranteed to lead to anything against competent players, but it's still an ok option if you're confident in reading your opponent. The main reason you don't see pikachus use downthrow often is because uthrow and bthrow are really good at getting guaranteed damage or positional advantage, and are thus more often used.
Welcome to the pika forums! If you're relatively new to competative melee, feel free to read my guide further down in the forum, and/or check out the Ask About Pikachu thread as it has a wealth of useful pikachu information. Also feel free to ask us anything else that you're curious about!
