Does a +2 from rumble scrapper is arithmetic or trait altering?

Arithmetic, not trait-altering — and to be precise, Rumble, Scrapper grants +1 Might to your Mechs, not +2.

CR 479.4.a puts increases and decreases to Might in the Arithmetic layer, which is where "+1 Might" from Rumble's passive applies. The Trait-Altering layer (477.1) handles things like Name, Type, Tags, and assignment of Might (e.g. "Might becomes 4"), not numeric bonuses. Since Rumble's bonus is an increase from a passive ability, it also never snapshots — it re-evaluates continuously per CR 479.4.b.

Practically, this matters for order of operations: because it's arithmetic, the +1 applies on top of any setting effect, and two sources of +1 (like a copy of Rumble) both stack.