No! They already had Hoskins and Bohm, who are defensive liabilities, who are best suited to DH. Ditto Schwarber. Castellanos may be the best defender of the four, but is, or surely will be within a couple seasons, another DH. Building a team with 4 DHs, coupled with a poor defender at SS and light bats in CF who are merely ok/good defensively is a poorly constructed roster.
I'm not a big Girardi fan, but the major problems lie elsewhere -- the biggest being the historical (possibly now fixed but several years until we see results?) but also the loading of the roster with DH's. I have absolutely no problem with Castellanos as an individual signing, but coupled with Schwarber and the guys we already had, it was bad.