Not sure if that is a smart move by Zelensky. He needs all the help he can get.
Steinmeier is of the SPD party that historically always had close ties to Russia and has struggled to act convincingly. It is no surprise that Ukraine/Zelensky is quite unhappy about the German contribution, as our military has essentially failed to deliver weapons in the way Ukraine needed and wanted.
And you can see a clear pattern looking at the current government:
Chancellor Olaf Scholz (SPD): Made one big speech, but isn't walking the walk so far (remember that "100 billion Euro investment in the military"? looks like he doesn't)
Minister of Defence Christine Lambrecht (SPD): Horrible performance in all this. Promised weapons out of Bundeswehr stock, didn't deliver, looks completely overwhelmed by actually having to deal with a war situation.
Minister of Economy Robert Habeck (Greens): Responsible for the first actually done weapon deliveries (Ukraine bought stuff directly from the manufacturer, he waived them through as he is responsible for weapon exports)
Foreign Minister Annalena Baerbock (Greens): Publicly calling for heavy weapons for Ukraine, despite the reluctance of Scholz to do so
Leader of the parliamentary defence committee Marie-Agnes Strack-Zimmermann (FDP): Calling for heavy weapons for Ukraine, publicly calling out chancellor Scholz for being a weak leader in the current crisis
So out of the three current governing parties, two are working and calling for heavy support for Ukraine, while the SPD is reluctant to do so and responsible for a lot of the appeasement politics towards Russia, Steinmeier as former foreign minister even so under Merkel's CDU government.
And by the way a level below German politicians are visiting Ukraine today, namely the leaders of the parliamentary committees for defence, Europe (Anton Hofreiter, Greens) and foreign politics (Michael Roth, SPD), but apparently they won't go to Kiew but meet with Ukrainian parlamentarians in Western Ukraine (which makes sense, as that is the same protocollary level).