The sentiment against recent-gen fire starters isn't universal, sure (just about no subjective opinion ever is), but it's definitely a sentiment I've observed from plenty of people as well, so I don't think it's fair to say the OP is making stuff up. If anything, the recent-gen fire starters have received quite a bit of vocal backlash lately, if not being disliked by a majority.
I'd say the increasing flack towards the more recent fire starters appears to be due to the designs being, for lack of a better word, flanderized in recent generations. In gen 3, blaziken was introduced as the first fire/fighting type, a novel type which was a pretty cool combination. And, of course, with the fighting type came some degree of anthropomorphization which is kind of a must for a fighting type, which has to feasibly pull off stylized fight moves, though in blaziken's case it was relatively subtle. This was followed up in gen 4 with another fire/fighting type, the first ever repeat of a dual type combination for a starter, and slightly more anthropomorphized than the last, with armour-esque extensions. Still, it was relatively subtle, and infernape definitely had quite a bit of appeal to it, so people still liked it.
Gen 5 seemed to be a turning point, and looking back, I'd say a lot of things came together to make emboar leave a bad taste in the mouth of many which would affect reception for fire type starters for a long time coming. First off, this was the first time any type, dual or single had been repeated 3 times in a row for 3 generations straight. I think a lot of players were ready for something new at that point, but the developers gave more of the same - and as the only dual typed starter in the whole gen to boot, making the repeat feel forced. The anthropomorphization in the design was also much more apparent here - monkeys are primates like humans, and birds are already bipedal like humans, so the anthropomorphization in the previous starters' designs was a bit more natural and could be played more subtly. Pigs, on the other hand, are quadrupedal, and even emboar's unevolved form was quadrupedal, so emboar came off a lot more egregiously anthropomorphic than its forebears - again, making the concept feel kind of forced, sort of shoehorning tepig into a starter archetype it didn't really fit in, attempting to recapture the magic of the previous starters with the same type. And I think the fact that emboar was just generally less appealing than the fire starters before it just capped off the disdain. Rather than the agile, acrobatic starters that came before it, emboar was a bulky, clunky thing - while somehow having worse defence stats than any of the leaner fire starters that came before it. Having little effectiveness in 2-player battles left emboar with little to like about it and quite a few people really wishing the fire starters to be taken in a new direction.
As the generations went on, though, the developers seem to have staunchly refused to do this, instead leaning directly into several of the trends which frustrated a lot of players. Ever since, the fire starters have been anthropomorphic mammals with 2 out of 3 of them evolving from quadrupedal unevolved forms and 2 out of 3 of them having designs which could feasibly be for fighting types - cinderace's jockish design makes it seem about as suited to the fighting type as the likes of passimian, and incineroar is based on a wrestler, much like emboar, and the fact that its design seems to deliberately incorporate a lot of emboar's disliked characteristics to troll the outcriers, while suiting its heel persona pretty well, probably didn't help the reputation of fire starters overall. I wouldn't necessarily say all these designs are bad (I probably like delphox the best out of Kalos's starters), and there are still quite a few who like these starters (incineroar was very popular among VGC players, and I've heard it said that cinderace was actually the most popular gen 8 starter overall), but there is a quite vocal crowd who strongly believes that the people who design the fire type starters are in a rut of anthropomorphic, usually sporty mammals. I've heard this sort of complaint regarding final forms of starters as a whole, that they're getting too anthropomorphized as of late (look at all the memes of people crossing their fingers for sprigatito to remain quadrupedal after evolving), but the fire starters appear to especially epitomize this, especially considering that it's the only starter type whose final forms have consistently had a humanoid body plan - there have been a few water starters which end up looking pinniped-like, and the grass starters had an early trend of being quadrupedal (and even the more human-character-inspired serperior is still serpentine).
Time will tell if the starters of the future will begin to look different. Sure these trends aren't objective design flaws, and there may not even actually be that many people who mind, but those who do have a bone to pick with the designs of fully-evolved starters, especially the fire-typed ones, have a vocal presence, and these are my best guesses as to why they aren't too happy with the later fire starters.
