Can the beetle crawl up more than one level in a single move? For example, if the beetle is on ground level (level zero), can it move up on top of stack of two pieces (i.e. onto level 2) in one turn? I feel like it shouldn't be allowed to move in top of a stack of n pieces (i.e. onto level n+1) unless it is already on level n, since an actual beetle cannot jump or fly... I'd appreciate an answer, thanks.
Yes it can jump multiple levels.
From the rules here it says as follows.
The only way to block a Beetle that is on top of the Hive is to move another Beetle on top of it. All four Beetles can be stacked on top of each other.
There is no reference in the rules to moving one level at a time. If a beetle was to move on top of a stack of all Beetles (as stated in the rules is possible) then the Beetle would have to jump (or climb if you feel that makes more thematic sense) up more than one level at once.