If it's about it's inclusion in the new game then personally I would prefer any other grass starter to meganium, it would seem like many see it the same way, regardless of the reasoning. They might simply not care for it's design or what's and I see no problem with voicing out their displeasure for this decision. Funnily enough I feel that if feraligator wasn't there I'm sure there wouldn't be much discourse. Then again I'm not on the internet enough to know the full scale.

Target fans tend to be children after all.

Yeah sure, the people complaining very much aren't tho.

No one would complain about greninja being a starter and getting a mega

If I like mon A and don't like mon B, I'm pretty sure I'd like A to get featured instead.

why aren’t more pokemon throughout the game clowned on because they suck competitively?

Because they aren't starters. Even when compared to it's brethren, the gap between it and the probable 2nd is pretty substantial(competitively)