It’s a bit hit or miss, depends on the context. On the one hand, calling someone out for the toon they post on could be construed as an ad hominem, distracting from the arguments presented by putting the other person on the defensive.
On the other hand, there are few reasons to post on a low level alt except to conceal who your main is, and by proxy conceal your actual experience. Since that experience is often relevant to the argument at hand, the act of concealing it can be validly inferred as shame over one’s lack of experience (why would you hide having that experience, after all?), and thus the act of posting on a low-level alt can be construed as a tacit admission that the person in question does not in fact have the credentials or experience they imply or state they have.
The only major reason people post on low level alts is to dissociate their posting behavior from their main, to avoid any reputation they gain from posting coming back and impacting their gameplay on their main. Since that’s really only a relevant desire when one’s posting behavior is highly negative (ie. “trolling”, though it’s broader than the actual definition of trolling, and includes unnecessarily antagonistic or incendiary posting styles), it’s not necessarily an untoward assumption that one posting on a low-level alt intends to be inflammatory or otherwise post ridiculous positions just for the enjoyment of inciting a flame-war.
On the balance, I tend to lean slightly towards the latter camp. The person posting made a conscious decision to use that low level character. While the fact that the post is coming from a low-level character does not necessarily belay the validity of the point being made, it does raise very valid and rather commonly applicable questions as to why they person chose to use a low-level alt for posting, and thus what their actual intentions on the forums happen to be.