Siding with the Rats on Impeachment of a Republican President is a peculiar way of showing his conservatism.
Fealty to a cult of personality and a party line is a peculiar way of showing conservatism, too. Especially when said President reneges on his promises, appoints liberals like Brett Kavanaugh to SCOTUS, blows up the federal deficit, and has a long record of questionable (to put it mildly) sexual conduct.
I disagree with Amash on this particular issue being impeachable, but that disagreement is personal, not related to his political views.