Contested Sequel: To the old-school lineage of Super Mario Bros games from the NES, SNES and (to a lesser extent) Game Boy; the primary reason is because many fans perceive an insufficient progress from the games in refreshing the gameplay formula. It didn't get this reaction too bad until Nintendo announced a third and fourth game to be released in the same year (namely 2012), and since then people began to deride Nintendo for over-relying on the sub-series, if not Mario as a whole, derisively being compared to the oversaturation of the Call of Duty series. For reference, New Super Mario Bros. 1 launched in 2006, and in less than a decade has now accumulated more installments and DLC than series that have been around since the N64 era, like Paper Mario or Super Smash Bros.. Though it hasn't stopped each game from being a commercial success, and while the critical reception has remained positive, fan reactions have been more polarized.
Dork Age: Devoted Mario fans consider the NSMB games to have defined a Dork Age for the franchise, despite (or even because of) their success. The catalyst was arguably New Super Mario Bros. Wii outselling Super Mario Galaxy due to the Wii's more casual-focused audience, causing the entire franchise to follow suit and copy the NSMB formula and its standards of "safeness" and shallowness for a time, most infamously with games like Paper Mario: Sticker Star.
It's Easy, So It Sucks: A common complaint of the sub-series, at least the handheld games. The console games are much more difficult.
It's the Same, Now It Sucks: Every game in the series invoked this reaction from people due to it being a 2D platformer with the same artstyle, which reached a peak with 2 and U.
"Seinfeld" Is Unfunny: The fanbase was pretty hyped for the original New Super Mario Bros., as it was the 2D Mario revival they had been clamoring for, and it got massive critical and commercial success upon release. It also helped revive 2D platformers on consoles. Not long after the release of New Super Mario Bros. Wii, othertitlesfollowed. Now, with new NSMB games having been released on a fairly regular basis, some fans have grown to view the sub-series as a pointless, boring series of rehashes, and often compare it unfavorably with other Mario titles (even accusing its "generic" formula of seeping into Mario games in other genres).