Managed to solve it myself! :)
Here's my solution for everyone with a similar problem:
%% Mean Bottom Portfolio
meanDS_10B = NaN(1,numel(DS_10B));
for k =1:numel(DS_10B)
t = DS_10B{k,:};
% Extract stock data columns of every table
lastColumn = t{:, 4:end}; % Get all columns with stock return data of table as column vectors of doubles
%reshape into one single column in order to calculate mean
lastCo = reshape(lastColumn,[],1);
% mean the last column but use omitnan
mean_column = mean(lastCo,'omitnan');
meanDS_10B(k) = mean_column;
end
% Average all Means
avgRET_Bottom = mean(meanDS_10B,'omitnan');