Does deathtouch apply when a creature deals damage to another creature via trample spillover?

Deathtouch is a static ability that means any amount of damage that creature deals to another creature is enough to destroy it (CR 702.2b). Crucially, this interacts with the combat damage assignment rules for trample (CR 702.19b), but only in how damage is assigned, not in whether deathtouch applies to what is actually dealt.

When a creature with both trample and deathtouch attacks, the controller only needs to assign lethal damage to each blocking creature before assigning the remainder as trample damage to the defending player or planeswalker. Because a creature with deathtouch causes even 1 point of damage to be considered lethal (CR 702.2b, 510.1c), the attacker can assign just 1 damage to each blocker and trample the rest over — this is a significant advantage.

The deathtouch ability itself then applies to the damage actually dealt to the blocking creature. Any blocking creature that receives even 1 damage from a deathtouch creature will be destroyed by the deathtouch triggered effect (state-based action, CR 704.5h). The spillover trample damage going to the player or planeswalker is simply normal damage — deathtouch has no meaning there, since players and planeswalkers are not destroyed by deathtouch.

Example: You attack with a 6/6 creature with trample and deathtouch. Your opponent blocks with a 5/5. You assign 1 damage to the 5/5 (lethal due to deathtouch) and 5 damage tramples through to the defending player. The 5/5 is destroyed by deathtouch, and the player takes 5 damage.

So yes, deathtouch fully applies to the blocked creatures taking trample damage, and the combination of trample + deathtouch is especially powerful because it minimizes how much damage must be assigned to blockers before trampling over.